Encoder, decoder, encoding method, and decoding method

ABSTRACT

An encoder which transforms a current block to be encoded in an image to encode the current block includes circuitry and memory. The circuitry, using the memory: determines a plurality of first transform basis candidates and transforms the current block using a transform basis included in the plurality of first transform basis candidates determined, when the current block has a first size; and determines one or more second transform basis candidates different from the plurality of first transform basis candidates and transforms the current block using a transform basis included in the one or more second transform basis candidates determined, when the current block has a second size larger than the first size.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. Continuation Application of PCT InternationalPatent Application Number PCT/JP2018/035903 filed on Sep. 27, 2018,claiming the benefit of priority of U.S. Provisional Patent ApplicationNo. 62/569,200 filed on Oct. 6, 2017, and the benefit of priority ofU.S. Patent Application No. 62/570,784 filed on Oct. 11, 2017, theentire contents of which are hereby incorporated by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to encoders, decoders, encoding methods,and decoding methods.

2. Description of the Related Art

Video coding standard called High-Efficiency Video Coding (HEVC) hasbeen standardized by Joint Collaborative Team on Video Coding (JCT-VC).See H.265 (ISO/IEC 23008-2 HEVC (High Efficiency Video Coding)).

SUMMARY

An encoder according to an aspect of the present disclosure is anencoder which transforms a current block to be encoded in an image toencode the current block includes circuitry and memory. The circuitry,using the memory: determines a plurality of first transform basiscandidates and transforms the current block using a transform basisincluded in the plurality of first transform basis candidatesdetermined, when the current block has a first size; and determines oneor more second transform basis candidates different from the pluralityof first transform basis candidates and transforms the current blockusing a transform basis included in the one or more second transformbasis candidates determined, when the current block has a second sizelarger than the first size.

A decoder according to an aspect of the present disclosure is a decoderwhich inverse-transforms a current block to be decoded in an encodedimage to decode the current block includes circuitry and memory. Thecircuitry, using the memory: determines a plurality of first inversetransform basis candidates and inverse-transforms the current blockusing an inverse transform basis included in the plurality of firstinverse transform basis candidates determined, when the current blockhas a first size; and determines one or more second inverse transformbasis candidates different from the plurality of first inverse transformbasis candidates and inverse-transforms the current block using aninverse transform basis included in the one or more inverse secondtransform basis candidates determined, when the current block has asecond size larger than the first size.

It is to be noted that these general and specific aspects may beimplemented using a system, a method, an integrated circuit, a computerprogram, or a computer-readable recording medium such as a CD-ROM, orany combination of systems, methods, integrated circuits, computerprograms, or computer-readable recording media.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, advantages and features of the disclosure willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the present disclosure.

FIG. 1 is a block diagram illustrating a functional configuration of anencoder according to Embodiment 1.

FIG. 2 illustrates one example of block splitting according toEmbodiment 1.

FIG. 3 is a chart indicating transform basis functions for eachtransform type.

FIG. 4A illustrates one example of a filter shape used in ALF.

FIG. 4B illustrates another example of a filter shape used in ALF.

FIG. 4C illustrates another example of a filter shape used in ALF.

FIG. 5A illustrates 67 intra prediction modes used in intra prediction.

FIG. 5B is a flow chart for illustrating an outline of a predictionimage correction process performed via OBMC processing.

FIG. 5C is a conceptual diagram for illustrating an outline of aprediction image correction process performed via OBMC processing.

FIG. 5D illustrates one example of FRUC.

FIG. 6 is for illustrating pattern matching (bilateral matching) betweentwo blocks along a motion trajectory.

FIG. 7 is for illustrating pattern matching (template matching) betweena template in the current picture and a block in a reference picture.

FIG. 8 is for illustrating a model assuming uniform linear motion.

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks.

FIG. 9B is for illustrating an outline of a process for deriving amotion vector via merge mode.

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing.

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing.

FIG. 10 is a block diagram illustrating a functional configuration of adecoder according to Embodiment 1.

FIG. 11 is a block diagram illustrating an internal structure of atransformer in the encoder according to Embodiment 1.

FIG. 12 is a diagram illustrating a plurality of examples of positionsof selected basis information, threshold size information, or transformmode information in a bitstream in Embodiment 1 or Variation 1 or 2.

FIG. 13 is a flowchart indicating operations performed by thetransformer in the encoder according to Embodiment 1.

FIG. 14 is a block diagram illustrating an internal structure of aninverse transformer in the decoder according to Embodiment 1.

FIG. 15 is a flowchart indicating operations performed by the inversetransformer in the decoder according to Embodiment 1.

FIG. 16 is a block diagram illustrating an internal structure of atransformer in an encoder according to Variation 1 of Embodiment 1.

FIG. 17 is a flowchart indicating operations performed by a transformerin the encoder according to Variation 1 of Embodiment 1.

FIG. 18 is a block diagram illustrating an internal structure of aninverse transformer in a decoder according to Variation 1 of Embodiment1.

FIG. 19 is a flowchart indicating operations performed by the inversetransformer in the decoder according to Variation 1 of Embodiment 1.

FIG. 20 is a block diagram illustrating an internal structure of atransformer in an encoder according to Variation 2 of Embodiment 1.

FIG. 21 is a flowchart indicating operations performed by thetransformer in the encoder according to Variation 2 of Embodiment 1.

FIG. 22 is a flowchart indicating operations performed by a basisselector in the encoder according to Variation 2 of Embodiment 1.

FIG. 23 is a block diagram illustrating an internal structure of atransformer in an encoder according to Variation 3 of Embodiment 1.

FIG. 24 is a flowchart indicating operations performed by thetransformer in the encoder according to Variation 3 of Embodiment 1.

FIG. 25 is a flowchart indicating operations performed by thetransformer in the encoder according to Variation 3 of Embodiment 1.

FIG. 26 is a block diagram illustrating an internal structure of aninverse transformer in a decoder according to Variation 3 of Embodiment1.

FIG. 27 is a flowchart indicating operations performed by the inversetransformer in the decoder according to Variation 3 of Embodiment 1.

FIG. 28 is a flowchart indicating operations performed by the inversetransformer in the decoder according to Variation 3 of Embodiment 1.

FIG. 29 is a flowchart indicating operations performed by a transformerof an encoder according to Variation 4 of Embodiment 1.

FIG. 30 is a diagram indicating transform bases which enable fastcomputation for block sizes.

FIG. 31 is a flowchart indicating operations performed by an inversetransformer of a decoder according to Variation 4 of Embodiment 1.

FIG. 32 is a flowchart indicating operations performed by a transformerof an encoder according to Variation 5 of Embodiment 1.

FIG. 33 is a flowchart indicating operations performed by an inversetransformer in a decoder according to Variation 5 of Embodiment 1.

FIG. 34 illustrates an overall configuration of a content providingsystem for implementing a content distribution service.

FIG. 35 illustrates one example of an encoding structure in scalableencoding.

FIG. 36 illustrates one example of an encoding structure in scalableencoding.

FIG. 37 illustrates an example of a display screen of a web page.

FIG. 38 illustrates an example of a display screen of a web page.

FIG. 39 illustrates one example of a smartphone.

FIG. 40 is a block diagram illustrating a configuration example of asmartphone.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(Underlying Knowledge Forming Basis of the Present Disclosure)

Methods for selectively using a plurality of bases in order toefficiently perform frequency transform on residuals of a current blockto be encoded have been proposed (examples of the methods includeexplicit multiple core transform (EMT) and adaptive multiple transform(AMT)). Such methods require evaluation (cost evaluation, etc.) of theplurality of bases in order to select a basis for a current block fromamong the plurality of bases, which increases the load and time for anencoding process. Furthermore, such methods require a computationcircuit for transform/inverse-transform using each basis, resulting inincrease in circuit scale. In addition, there are some bases which donot enable fast computation depending on block sizes, and thusprocessing time increases when a basis that does not enable fastcomputation is selected.

(Summary of the Present Disclosure)

In view of this, an encoder according to an aspect of the presentdisclosure is an encoder which transforms a current block to be encodedin an image to encode the current block includes circuitry and memory.The circuitry, using the memory: determines a plurality of firsttransform basis candidates and transforms the current block using atransform basis included in the plurality of first transform basiscandidates determined, when the current block has a first size; anddetermines one or more second transform basis candidates different fromthe plurality of first transform basis candidates and transforms thecurrent block using a transform basis included in the one or more secondtransform basis candidates determined, when the current block has asecond size larger than the first size.

With the encoder, it is possible to switch transform basis candidatesdepending on a block size of a current block, and thus to use a moresuitable transform basis. For example, it is possible to reduce cost forsignalling transform basis information and reduce processing load and/orprocessing time if reducing the number of transform basis candidateswhen the current block has the second size larger than the first size.

For example, in the encoder according to the aspect of the presentdisclosure, the number of the one or more second transform basiscandidates may be smaller than the number of the plurality of firsttransform basis candidates.

In this way, it is possible to reduce the number of transform basiscandidates when the current block has the second size larger than thefirst size, and thus to reduce cost for signalling the transform basisinformation and reduce the number of transform coefficients byadaptively selecting the transform basis from the plurality of transformbasis candidates. Furthermore, it is also possible to exclude at leastone transform basis from the transform basis candidates based on acomputation amount, and thus to reduce processing load and/or processingtime.

For example, in the encoder according to the aspect of the presentdisclosure, each of the one or more second transform basis candidatesmay be included in the plurality of first transform basis candidates.

In this way, the plurality of first transform basis candidates caninclude the one or more second transform basis candidates. In otherwords, it is possible to prepare the one or more second transform basiscandidates by excluding at least one transform basis from the pluralityof first transform basis candidates. For example, when a block size islarge, it is possible to prevent a transform basis which requires alarge computation amount from being used by excluding the transformbasis which requires the large computation amount from the firsttransform basis candidates. In this way, it is possible to reduceprocessing load and/or processing time more effectively. In addition, itis also possible to reduce the circuit scale of a dedicated circuit byexcluding the transform basis which requires the large computationamount which is used for a large block size from the transform basiscandidates.

Furthermore, a decoder according to an aspect of the present disclosureis a decoder which inverse-transforms a current block to be decoded inan encoded image to decode the current block includes circuitry andmemory. The circuitry, using the memory: determines a plurality of firstinverse transform basis candidates and inverse-transforms the currentblock using an inverse transform basis included in the plurality offirst inverse transform basis candidates determined, when the currentblock has a first size; and determines one or more second inversetransform basis candidates different from the plurality of first inversetransform basis candidates and inverse-transforms the current blockusing an inverse transform basis included in the one or more inversesecond transform basis candidates determined, when the current block hasa second size larger than the first size.

With the decoder, it is possible to switch inverse transform basiscandidates depending on the block size of the current block, and thus touse the more suitable inverse transform basis. For example, it ispossible to reduce cost for signalling transform basis information ifreducing the number of inverse transform basis candidates when thecurrent block has the second size larger than the first size.

For example, in the decoder according to the aspect of the presentdisclosure, the number of the one or more second inverse transform basiscandidates may be smaller than the number of the plurality of firstinverse transform basis candidates.

In this way, it is possible to reduce the number of inverse transformbasis candidates when the current block has the second size larger thanthe first size, and thus can reduce cost for signalling transform basisinformation and reduce the number of transform coefficients.Furthermore, it is also possible to exclude at least one inversetransform basis from inverse transform basis candidates based on acomputation amount, and thus to reduce processing load and/or processingtime.

For example, in the decoder according to the aspect of the presentdisclosure, each of the one or more second inverse transform basiscandidates may be included in the plurality of first inverse transformbasis candidates.

In this way, the plurality of first inverse transform basis candidatescan include the one or more second inverse transform basis candidates.In other words, it is possible to prepare the one or more second inversetransform basis candidates by excluding the at least one inversetransform basis from the plurality of first inverse transform basiscandidates. For example, when the block size of a current block islarge, it is possible to prevent an inverse transform basis whichrequires a large computation amount from being used by excluding theinverse transform basis which requires the large computation amount fromthe first inverse transform basis candidates. In this way, it ispossible to reduce processing load and/or processing time moreeffectively. In addition, it is also possible to reduce the circuitscale of a dedicated circuit by excluding the inverse transform basiswhich requires the large computation amount which is used for a largeblock size from the first inverse transform basis candidates.

It is to be noted that these general and specific aspects may beimplemented using a system, a method, an integrated circuit, a computerprogram, or a computer-readable recording medium such as a CD-ROM, orany combination of systems, methods, integrated circuits, computerprograms, or computer-readable recording media.

Embodiment 1

First, an outline of Embodiment 1 will be presented. Embodiment 1 is oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in subsequent description of aspects of thepresent disclosure are applicable. Note that Embodiment 1 is merely oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in the description of aspects of the presentdisclosure are applicable. The processes and/or configurations presentedin the description of aspects of the present disclosure can also beimplemented in an encoder and a decoder different from those accordingto Embodiment 1.

When the processes and/or configurations presented in the description ofaspects of the present disclosure are applied to Embodiment 1, forexample, any of the following may be performed.

(1) regarding the encoder or the decoder according to Embodiment 1,among components included in the encoder or the decoder according toEmbodiment 1, substituting a component corresponding to a componentpresented in the description of aspects of the present disclosure with acomponent presented in the description of aspects of the presentdisclosure;

(2) regarding the encoder or the decoder according to Embodiment 1,implementing discretionary changes to functions or implemented processesperformed by one or more components included in the encoder or thedecoder according to Embodiment 1, such as addition, substitution, orremoval, etc., of such functions or implemented processes, thensubstituting a component corresponding to a component presented in thedescription of aspects of the present disclosure with a componentpresented in the description of aspects of the present disclosure;

(3) regarding the method implemented by the encoder or the decoderaccording to Embodiment 1, implementing discretionary changes such asaddition of processes and/or substitution, removal of one or more of theprocesses included in the method, and then substituting a processescorresponding to a process presented in the description of aspects ofthe present disclosure with a process presented in the description ofaspects of the present disclosure;

(4) combining one or more components included in the encoder or thedecoder according to Embodiment 1 with a component presented in thedescription of aspects of the present disclosure, a component includingone or more functions included in a component presented in thedescription of aspects of the present disclosure, or a component thatimplements one or more processes implemented by a component presented inthe description of aspects of the present disclosure;

(5) combining a component including one or more functions included inone or more components included in the encoder or the decoder accordingto Embodiment 1, or a component that implements one or more processesimplemented by one or more components included in the encoder or thedecoder according to Embodiment 1 with a component presented in thedescription of aspects of the present disclosure, a component includingone or more functions included in a component presented in thedescription of aspects of the present disclosure, or a component thatimplements one or more processes implemented by a component presented inthe description of aspects of the present disclosure;

(6) regarding the method implemented by the encoder or the decoderaccording to Embodiment 1, among processes included in the method,substituting a process corresponding to a process presented in thedescription of aspects of the present disclosure with a processpresented in the description of aspects of the present disclosure; and

(7) combining one or more processes included in the method implementedby the encoder or the decoder according to Embodiment 1 with a processpresented in the description of aspects of the present disclosure.

Note that the implementation of the processes and/or configurationspresented in the description of aspects of the present disclosure is notlimited to the above examples. For example, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be implemented in a device used for a purpose differentfrom the moving picture/picture encoder or the moving picture/picturedecoder disclosed in Embodiment 1. Moreover, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be independently implemented. Moreover, processes and/orconfigurations described in different aspects may be combined.

[Encoder Outline]

First, the encoder according to Embodiment 1 will be outlined. FIG. 1 isa block diagram illustrating a functional configuration of encoder 100according to Embodiment 1. Encoder 100 is a moving picture/pictureencoder that encodes a moving picture/picture block by block.

As illustrated in FIG. 1, encoder 100 is a device that encodes a pictureblock by block, and includes splitter 102, subtractor 104, transformer106, quantizer 108, entropy encoder 110, inverse quantizer 112, inversetransformer 114, adder 116, block memory 118, loop filter 120, framememory 122, intra predictor 124, inter predictor 126, and predictioncontroller 128.

Encoder 100 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as splitter 102, subtractor104, transformer 106, quantizer 108, entropy encoder 110, inversequantizer 112, inverse transformer 114, adder 116, loop filter 120,intra predictor 124, inter predictor 126, and prediction controller 128.Alternatively, encoder 100 may be realized as one or more dedicatedelectronic circuits corresponding to splitter 102, subtractor 104,transformer 106, quantizer 108, entropy encoder 110, inverse quantizer112, inverse transformer 114, adder 116, loop filter 120, intrapredictor 124, inter predictor 126, and prediction controller 128.

Hereinafter, each component included in encoder 100 will be described.

[Splitter]

Splitter 102 splits each picture included in an input moving pictureinto blocks, and outputs each block to subtractor 104. For example,splitter 102 first splits a picture into blocks of a fixed size (forexample, 128×128). The fixed size block is also referred to as codingtree unit (CTU). Splitter 102 then splits each fixed size block intoblocks of variable sizes (for example, 64×64 or smaller), based onrecursive quadtree and/or binary tree block splitting. The variable sizeblock is also referred to as a coding unit (CU), a prediction unit (PU),or a transform unit (TU). Note that in this embodiment, there is no needto differentiate between CU, PU, and TU; all or some of the blocks in apicture may be processed per CU, PU, or TU.

FIG. 2 illustrates one example of block splitting according toEmbodiment 1. In FIG. 2, the solid lines represent block boundaries ofblocks split by quadtree block splitting, and the dashed lines representblock boundaries of blocks split by binary tree block splitting.

Here, block 10 is a square 128×128 pixel block (128×128 block). This128×128 block 10 is first split into four square 64×64 blocks (quadtreeblock splitting).

The top left 64×64 block is further vertically split into two rectangle32×64 blocks, and the left 32×64 block is further vertically split intotwo rectangle 16×64 blocks (binary tree block splitting). As a result,the top left 64×64 block is split into two 16×64 blocks 11 and 12 andone 32×64 block 13.

The top right 64×64 block is horizontally split into two rectangle 64×32blocks 14 and 15 (binary tree block splitting).

The bottom left 64×64 block is first split into four square 32×32 blocks(quadtree block splitting). The top left block and the bottom rightblock among the four 32×32 blocks are further split. The top left 32×32block is vertically split into two rectangle 16×32 blocks, and the right16×32 block is further horizontally split into two 16×16 blocks (binarytree block splitting). The bottom right 32×32 block is horizontallysplit into two 32×16 blocks (binary tree block splitting). As a result,the bottom left 64×64 block is split into 16×32 block 16, two 16×16blocks 17 and 18, two 32×32 blocks 19 and 20, and two 32×16 blocks 21and 22.

The bottom right 64×64 block 23 is not split.

As described above, in FIG. 2, block 10 is split into 13 variable sizeblocks 11 through 23 based on recursive quadtree and binary tree blocksplitting. This type of splitting is also referred to as quadtree plusbinary tree (QTBT) splitting.

Note that in FIG. 2, one block is split into four or two blocks(quadtree or binary tree block splitting), but splitting is not limitedto this example. For example, one block may be split into three blocks(ternary block splitting). Splitting including such ternary blocksplitting is also referred to as multi-type tree (MBT) splitting.

[Subtractor]

Subtractor 104 subtracts a prediction signal (prediction sample) from anoriginal signal (original sample) per block split by splitter 102. Inother words, subtractor 104 calculates prediction errors (also referredto as residuals) of a block to be encoded (hereinafter referred to as acurrent block). Subtractor 104 then outputs the calculated predictionerrors to transformer 106.

The original signal is a signal input into encoder 100, and is a signalrepresenting an image for each picture included in a moving picture (forexample, a luma signal and two chroma signals). Hereinafter, a signalrepresenting an image is also referred to as a sample.

[Transformer]

Transformer 106 transforms spatial domain prediction errors intofrequency domain transform coefficients, and outputs the transformcoefficients to quantizer 108. More specifically, transformer 106applies, for example, a predefined discrete cosine transform (DCT) ordiscrete sine transform (DST) to spatial domain prediction errors.

Note that transformer 106 may adaptively select a transform type fromamong a plurality of transform types, and transform prediction errorsinto transform coefficients by using a transform basis functioncorresponding to the selected transform type. This sort of transform isalso referred to as explicit multiple core transform (EMT) or adaptivemultiple transform (AMT).

The transform types include, for example, DCT-II, DCT-V, DCT-VIII,DST-I, and DST-VII. FIG. 3 is a chart indicating transform basisfunctions for each transform type. In FIG. 3, N indicates the number ofinput pixels. For example, selection of a transform type from among theplurality of transform types may depend on the prediction type (intraprediction and inter prediction), and may depend on intra predictionmode.

Information indicating whether to apply such EMT or AMT (referred to as,for example, an AMT flag) and information indicating the selectedtransform type is signalled at the CU level. Note that the signaling ofsuch information need not be performed at the CU level, and may beperformed at another level (for example, at the sequence level, picturelevel, slice level, tile level, or CTU level).

Moreover, transformer 106 may apply a secondary transform to thetransform coefficients (transform result). Such a secondary transform isalso referred to as adaptive secondary transform (AST) or non-separablesecondary transform (NSST). For example, transformer 106 applies asecondary transform to each sub-block (for example, each 4×4 sub-block)included in the block of the transform coefficients corresponding to theintra prediction errors. Information indicating whether to apply NSSTand information related to the transform matrix used in NSST aresignalled at the CU level. Note that the signaling of such informationneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, or CTU level).

Here, a separable transform is a method in which a transform isperformed a plurality of times by separately performing a transform foreach direction according to the number of dimensions input. Anon-separable transform is a method of performing a collective transformin which two or more dimensions in a multidimensional input arecollectively regarded as a single dimension.

In one example of a non-separable transform, when the input is a 4×4block, the 4×4 block is regarded as a single array including 16components, and the transform applies a 16×16 transform matrix to thearray.

Moreover, similar to above, after an input 4×4 block is regarded as asingle array including 16 components, a transform that performs aplurality of Givens rotations on the array (i.e., a Hypercube-GivensTransform) is also one example of a non-separable transform.

[Quantizer]

Quantizer 108 quantizes the transform coefficients output fromtransformer 106. More specifically, quantizer 108 scans, in apredetermined scanning order, the transform coefficients of the currentblock, and quantizes the scanned transform coefficients based onquantization parameters (QP) corresponding to the transformcoefficients. Quantizer 108 then outputs the quantized transformcoefficients (hereinafter referred to as quantized coefficients) of thecurrent block to entropy encoder 110 and inverse quantizer 112.

A predetermined order is an order for quantizing/inverse quantizingtransform coefficients. For example, a predetermined scanning order isdefined as ascending order of frequency (from low to high frequency) ordescending order of frequency (from high to low frequency).

A quantization parameter is a parameter defining a quantization stepsize (quantization width). For example, if the value of the quantizationparameter increases, the quantization step size also increases. In otherwords, if the value of the quantization parameter increases, thequantization error increases.

[Entropy Encoder]

Entropy encoder 110 generates an encoded signal (encoded bitstream) byvariable length encoding quantized coefficients, which are inputs fromquantizer 108. More specifically, entropy encoder 110, for example,binarizes quantized coefficients and arithmetic encodes the binarysignal.

[Inverse Quantizer]

Inverse quantizer 112 inverse quantizes quantized coefficients, whichare inputs from quantizer 108. More specifically, inverse quantizer 112inverse quantizes, in a predetermined scanning order, quantizedcoefficients of the current block. Inverse quantizer 112 then outputsthe inverse quantized transform coefficients of the current block toinverse transformer 114.

[Inverse Transformer]

Inverse transformer 114 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 112. More specifically, inverse transformer 114 restores theprediction errors of the current block by applying an inverse transformcorresponding to the transform applied by transformer 106 on thetransform coefficients. Inverse transformer 114 then outputs therestored prediction errors to adder 116.

Note that since information is lost in quantization, the restoredprediction errors do not match the prediction errors calculated bysubtractor 104. In other words, the restored prediction errors includequantization errors.

[Adder]

Adder 116 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 114, and prediction samples,which are inputs from prediction controller 128. Adder 116 then outputsthe reconstructed block to block memory 118 and loop filter 120. Areconstructed block is also referred to as a local decoded block.

[Block Memory]

Block memory 118 is storage for storing blocks in a picture to beencoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 118 storesreconstructed blocks output from adder 116.

[Loop Filter]

Loop filter 120 applies a loop filter to blocks reconstructed by adder116, and outputs the filtered reconstructed blocks to frame memory 122.A loop filter is a filter used in an encoding loop (in-loop filter), andincludes, for example, a deblocking filter (DF), a sample adaptiveoffset (SAO), and an adaptive loop filter (ALF).

In ALF, a least square error filter for removing compression artifactsis applied. For example, one filter from among a plurality of filters isselected for each 2×2 sub-block in the current block based on directionand activity of local gradients, and is applied.

More specifically, first, each sub-block (for example, each 2×2sub-block) is categorized into one out of a plurality of classes (forexample, 15 or 25 classes). The classification of the sub-block is basedon gradient directionality and activity. For example, classificationindex C is derived based on gradient directionality D (for example, 0 to2 or 0 to 4) and gradient activity A (for example, 0 to 4) (for example,C=5D+A). Then, based on classification index C, each sub-block iscategorized into one out of a plurality of classes (for example, 15 or25 classes).

For example, gradient directionality D is calculated by comparinggradients of a plurality of directions (for example, the horizontal,vertical, and two diagonal directions). Moreover, for example, gradientactivity A is calculated by summing gradients of a plurality ofdirections and quantizing the sum.

The filter to be used for each sub-block is determined from among theplurality of filters based on the result of such categorization.

The filter shape to be used in ALF is, for example, a circular symmetricfilter shape. FIG. 4A through FIG. 4C illustrate examples of filtershapes used in ALF. FIG. 4A illustrates a 5×5 diamond shape filter, FIG.4B illustrates a 7×7 diamond shape filter, and FIG. 4C illustrates a 9×9diamond shape filter. Information indicating the filter shape issignalled at the picture level. Note that the signaling of informationindicating the filter shape need not be performed at the picture level,and may be performed at another level (for example, at the sequencelevel, slice level, tile level, CTU level, or CU level).

The enabling or disabling of ALF is determined at the picture level orCU level. For example, for luma, the decision to apply ALF or not isdone at the CU level, and for chroma, the decision to apply ALF or notis done at the picture level. Information indicating whether ALF isenabled or disabled is signalled at the picture level or CU level. Notethat the signaling of information indicating whether ALF is enabled ordisabled need not be performed at the picture level or CU level, and maybe performed at another level (for example, at the sequence level, slicelevel, tile level, or CTU level).

The coefficients set for the plurality of selectable filters (forexample, 15 or 25 filters) is signalled at the picture level. Note thatthe signaling of the coefficients set need not be performed at thepicture level, and may be performed at another level (for example, atthe sequence level, slice level, tile level, CTU level, CU level, orsub-block level).

[Frame Memory]

Frame memory 122 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 122 stores reconstructed blocks filtered byloop filter 120.

[Intra Predictor]

Intra predictor 124 generates a prediction signal (intra predictionsignal) by intra predicting the current block with reference to a blockor blocks in the current picture and stored in block memory 118 (alsoreferred to as intra frame prediction). More specifically, intrapredictor 124 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 128.

For example, intra predictor 124 performs intra prediction by using onemode from among a plurality of predefined intra prediction modes. Theintra prediction modes include one or more non-directional predictionmodes and a plurality of directional prediction modes.

The one or more non-directional prediction modes include, for example,planar prediction mode and DC prediction mode defined in theH.265/high-efficiency video coding (HEVC) standard (see NPL 1).

The plurality of directional prediction modes include, for example, the33 directional prediction modes defined in the H.265/HEVC standard. Notethat the plurality of directional prediction modes may further include32 directional prediction modes in addition to the 33 directionalprediction modes (for a total of 65 directional prediction modes). FIG.5A illustrates 67 intra prediction modes used in intra prediction (twonon-directional prediction modes and 65 directional prediction modes).The solid arrows represent the 33 directions defined in the H.265/HEVCstandard, and the dashed arrows represent the additional 32 directions.

Note that a luma block may be referenced in chroma block intraprediction. In other words, a chroma component of the current block maybe predicted based on a luma component of the current block. Such intraprediction is also referred to as cross-component linear model (CCLM)prediction. Such a chroma block intra prediction mode that references aluma block (referred to as, for example, CCLM mode) may be added as oneof the chroma block intra prediction modes.

Intra predictor 124 may correct post-intra-prediction pixel values basedon horizontal/vertical reference pixel gradients. Intra predictionaccompanied by this sort of correcting is also referred to as positiondependent intra prediction combination (PDPC). Information indicatingwhether to apply PDPC or not (referred to as, for example, a PDPC flag)is, for example, signalled at the CU level. Note that the signaling ofthis information need not be performed at the CU level, and may beperformed at another level (for example, on the sequence level, picturelevel, slice level, tile level, or CTU level).

[Inter Predictor]

Inter predictor 126 generates a prediction signal (inter predictionsignal) by inter predicting the current block with reference to a blockor blocks in a reference picture, which is different from the currentpicture and is stored in frame memory 122 (also referred to as interframe prediction). Inter prediction is performed per current block orper sub-block (for example, per 4×4 block) in the current block. Forexample, inter predictor 126 performs motion estimation in a referencepicture for the current block or sub-block. Inter predictor 126 thengenerates an inter prediction signal of the current block or sub-blockby motion compensation by using motion information (for example, amotion vector) obtained from motion estimation. Inter predictor 126 thenoutputs the generated inter prediction signal to prediction controller128.

The motion information used in motion compensation is signalled. Amotion vector predictor may be used for the signaling of the motionvector. In other words, the difference between the motion vector and themotion vector predictor may be signalled.

Note that the inter prediction signal may be generated using motioninformation for a neighboring block in addition to motion informationfor the current block obtained from motion estimation. Morespecifically, the inter prediction signal may be generated per sub-blockin the current block by calculating a weighted sum of a predictionsignal based on motion information obtained from motion estimation and aprediction signal based on motion information for a neighboring block.Such inter prediction (motion compensation) is also referred to asoverlapped block motion compensation (OBMC).

In such an OBMC mode, information indicating sub-block size for OBMC(referred to as, for example, OBMC block size) is signalled at thesequence level. Moreover, information indicating whether to apply theOBMC mode or not (referred to as, for example, an OBMC flag) issignalled at the CU level. Note that the signaling of such informationneed not be performed at the sequence level and CU level, and may beperformed at another level (for example, at the picture level, slicelevel, tile level, CTU level, or sub-block level).

Hereinafter, the OBMC mode will be described in further detail. FIG. 5Bis a flowchart and FIG. 5C is a conceptual diagram for illustrating anoutline of a prediction image correction process performed via OBMCprocessing.

First, a prediction image (Pred) is obtained through typical motioncompensation using a motion vector (MV) assigned to the current block.

Next, a prediction image (Pred_L) is obtained by applying a motionvector (MV_L) of the encoded neighboring left block to the currentblock, and a first pass of the correction of the prediction image ismade by superimposing the prediction image and Pred_L.

Similarly, a prediction image (Pred_U) is obtained by applying a motionvector (MV_U) of the encoded neighboring upper block to the currentblock, and a second pass of the correction of the prediction image ismade by superimposing the prediction image resulting from the first passand Pred_U. The result of the second pass is the final prediction image.

Note that the above example is of a two-pass correction method using theneighboring left and upper blocks, but the method may be a three-pass orhigher correction method that also uses the neighboring right and/orlower block.

Note that the region subject to superimposition may be the entire pixelregion of the block, and, alternatively, may be a partial block boundaryregion.

Note that here, the prediction image correction process is described asbeing based on a single reference picture, but the same applies when aprediction image is corrected based on a plurality of referencepictures. In such a case, after corrected prediction images resultingfrom performing correction based on each of the reference pictures areobtained, the obtained corrected prediction images are furthersuperimposed to obtain the final prediction image.

Note that the unit of the current block may be a prediction block and,alternatively, may be a sub-block obtained by further dividing theprediction block.

One example of a method for determining whether to implement OBMCprocessing is by using an obmc_flag, which is a signal that indicateswhether to implement OBMC processing. As one specific example, theencoder determines whether the current block belongs to a regionincluding complicated motion. The encoder sets the obmc_flag to a valueof “1” when the block belongs to a region including complicated motionand implements OBMC processing when encoding, and sets the obmc_flag toa value of “0” when the block does not belong to a region includingcomplication motion and encodes without implementing OBMC processing.The decoder switches between implementing OBMC processing or not bydecoding the obmc_flag written in the stream and performing the decodingin accordance with the flag value.

Note that the motion information may be derived on the decoder sidewithout being signalled. For example, a merge mode defined in theH.265/HEVC standard may be used. Moreover, for example, the motioninformation may be derived by performing motion estimation on thedecoder side. In this case, motion estimation is performed without usingthe pixel values of the current block.

Here, a mode for performing motion estimation on the decoder side willbe described. A mode for performing motion estimation on the decoderside is also referred to as pattern matched motion vector derivation(PMMVD) mode or frame rate up-conversion (FRUC) mode.

One example of FRUC processing is illustrated in FIG. 5D. First, acandidate list (a candidate list may be a merge list) of candidates eachincluding a motion vector predictor is generated with reference tomotion vectors of encoded blocks that spatially or temporally neighborthe current block. Next, the best candidate MV is selected from among aplurality of candidate MVs registered in the candidate list. Forexample, evaluation values for the candidates included in the candidatelist are calculated and one candidate is selected based on thecalculated evaluation values.

Next, a motion vector for the current block is derived from the motionvector of the selected candidate. More specifically, for example, themotion vector for the current block is calculated as the motion vectorof the selected candidate (best candidate MV), as-is. Alternatively, themotion vector for the current block may be derived by pattern matchingperformed in the vicinity of a position in a reference picturecorresponding to the motion vector of the selected candidate. In otherwords, when the vicinity of the best candidate MV is searched via thesame method and an MV having a better evaluation value is found, thebest candidate MV may be updated to the MV having the better evaluationvalue, and the MV having the better evaluation value may be used as thefinal MV for the current block. Note that a configuration in which thisprocessing is not implemented is also acceptable.

The same processes may be performed in cases in which the processing isperformed in units of sub-blocks.

Note that an evaluation value is calculated by calculating thedifference in the reconstructed image by pattern matching performedbetween a region in a reference picture corresponding to a motion vectorand a predetermined region. Note that the evaluation value may becalculated by using some other information in addition to thedifference.

The pattern matching used is either first pattern matching or secondpattern matching. First pattern matching and second pattern matching arealso referred to as bilateral matching and template matching,respectively.

In the first pattern matching, pattern matching is performed between twoblocks along the motion trajectory of the current block in two differentreference pictures. Therefore, in the first pattern matching, a regionin another reference picture conforming to the motion trajectory of thecurrent block is used as the predetermined region for theabove-described calculation of the candidate evaluation value.

FIG. 6 is for illustrating one example of pattern matching (bilateralmatching) between two blocks along a motion trajectory. As illustratedin FIG. 6, in the first pattern matching, two motion vectors (MV0, MV1)are derived by finding the best match between two blocks along themotion trajectory of the current block (Cur block) in two differentreference pictures (Ref0, Ref1). More specifically, a difference between(i) a reconstructed image in a specified position in a first encodedreference picture (Ref0) specified by a candidate MV and (ii) areconstructed picture in a specified position in a second encodedreference picture (Ref1) specified by a symmetrical MV scaled at adisplay time interval of the candidate MV may be derived, and theevaluation value for the current block may be calculated by using thederived difference. The candidate MV having the best evaluation valueamong the plurality of candidate MVs may be selected as the final MV.

Under the assumption of continuous motion trajectory, the motion vectors(MV0, MV1) pointing to the two reference blocks shall be proportional tothe temporal distances (TD0, TD1) between the current picture (Cur Pic)and the two reference pictures (Ref0, Ref1). For example, when thecurrent picture is temporally between the two reference pictures and thetemporal distance from the current picture to the two reference picturesis the same, the first pattern matching derives a mirror basedbi-directional motion vector.

In the second pattern matching, pattern matching is performed between atemplate in the current picture (blocks neighboring the current block inthe current picture (for example, the top and/or left neighboringblocks)) and a block in a reference picture. Therefore, in the secondpattern matching, a block neighboring the current block in the currentpicture is used as the predetermined region for the above-describedcalculation of the candidate evaluation value.

FIG. 7 is for illustrating one example of pattern matching (templatematching) between a template in the current picture and a block in areference picture. As illustrated in FIG. 7, in the second patternmatching, a motion vector of the current block is derived by searching areference picture (Ref0) to find the block that best matches neighboringblocks of the current block (Cur block) in the current picture (CurPic). More specifically, a difference between (i) a reconstructed imageof an encoded region that is both or one of the neighboring left andneighboring upper region and (ii) a reconstructed picture in the sameposition in an encoded reference picture (Ref0) specified by a candidateMV may be derived, and the evaluation value for the current block may becalculated by using the derived difference. The candidate MV having thebest evaluation value among the plurality of candidate MVs may beselected as the best candidate MV.

Information indicating whether to apply the FRUC mode or not (referredto as, for example, a FRUC flag) is signalled at the CU level. Moreover,when the FRUC mode is applied (for example, when the FRUC flag is set totrue), information indicating the pattern matching method (first patternmatching or second pattern matching) is signalled at the CU level. Notethat the signaling of such information need not be performed at the CUlevel, and may be performed at another level (for example, at thesequence level, picture level, slice level, tile level, CTU level, orsub-block level).

Here, a mode for deriving a motion vector based on a model assuminguniform linear motion will be described. This mode is also referred toas a bi-directional optical flow (BIO) mode.

FIG. 8 is for illustrating a model assuming uniform linear motion. InFIG. 8, (v_(x), v_(y)) denotes a velocity vector, and τ₀ and τ₁ denotetemporal distances between the current picture (Cur Pic) and tworeference pictures (Ref₀, Ref₁). (MVx₀, MVy₀) denotes a motion vectorcorresponding to reference picture Ref₀, and (MVx₁, MVy₁) denotes amotion vector corresponding to reference picture Ref₁.

Here, under the assumption of uniform linear motion exhibited byvelocity vector (v_(x), v_(y)), (MVx₀, MVy₀) and (MVx₁, MVy₁) arerepresented as (v_(x)τ₀, v_(y)τ₀) and (−v_(x)τ₁, −v_(y)τ₁),respectively, and the following optical flow equation is given.MATH. 1∂I ^((k)) /∂t+v _(x) ∂I ^((k)) ∂x+v _(y) ∂I ^((k)) /∂y=0.  (1)Here, I^((k)) denotes a luma value from reference picture k (k=0, 1)after motion compensation. This optical flow equation shows that the sumof (i) the time derivative of the luma value, (ii) the product of thehorizontal velocity and the horizontal component of the spatial gradientof a reference picture, and (iii) the product of the vertical velocityand the vertical component of the spatial gradient of a referencepicture is equal to zero. A motion vector of each block obtained from,for example, a merge list is corrected pixel by pixel based on acombination of the optical flow equation and Hermite interpolation.

Note that a motion vector may be derived on the decoder side using amethod other than deriving a motion vector based on a model assuminguniform linear motion. For example, a motion vector may be derived foreach sub-block based on motion vectors of neighboring blocks.

Here, a mode in which a motion vector is derived for each sub-blockbased on motion vectors of neighboring blocks will be described. Thismode is also referred to as affine motion compensation prediction mode.

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks. In FIG. 9A, the currentblock includes 16 4×4 sub-blocks. Here, motion vector v₀ of the top leftcorner control point in the current block is derived based on motionvectors of neighboring sub-blocks, and motion vector v₁ of the top rightcorner control point in the current block is derived based on motionvectors of neighboring blocks. Then, using the two motion vectors v₀ andv₁, the motion vector (v_(x), v_(y)) of each sub-block in the currentblock is derived using Equation 2 below.

$\begin{matrix}{{MATH}.\mspace{14mu} 2} & \; \\\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{w}x} - {\frac{\left( {v_{1y} - v_{0y}} \right)}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0_{y}}} \right)}{w}x} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{w}y} + v_{0y}}}\end{matrix} \right. & (2)\end{matrix}$

Here, x and y are the horizontal and vertical positions of thesub-block, respectively, and w is a predetermined weighted coefficient.

Such an affine motion compensation prediction mode may include a numberof modes of different methods of deriving the motion vectors of the topleft and top right corner control points. Information indicating such anaffine motion compensation prediction mode (referred to as, for example,an affine flag) is signalled at the CU level. Note that the signaling ofinformation indicating the affine motion compensation prediction modeneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, CTU level, or sub-block level).

[Prediction Controller]

Prediction controller 128 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto subtractor 104 and adder 116.

Here, an example of deriving a motion vector via merge mode in a currentpicture will be given. FIG. 9B is for illustrating an outline of aprocess for deriving a motion vector via merge mode.

First, an MV predictor list in which candidate MV predictors areregistered is generated. Examples of candidate MV predictors include:spatially neighboring MV predictors, which are MVs of encoded blockspositioned in the spatial vicinity of the current block; a temporallyneighboring MV predictor, which is an MV of a block in an encodedreference picture that neighbors a block in the same location as thecurrent block; a combined MV predictor, which is an MV generated bycombining the MV values of the spatially neighboring MV predictor andthe temporally neighboring MV predictor; and a zero MV predictor, whichis an MV whose value is zero.

Next, the MV of the current block is determined by selecting one MVpredictor from among the plurality of MV predictors registered in the MVpredictor list.

Furthermore, in the variable-length encoder, a merge_idx, which is asignal indicating which MV predictor is selected, is written and encodedinto the stream.

Note that the MV predictors registered in the MV predictor listillustrated in FIG. 9B constitute one example. The number of MVpredictors registered in the MV predictor list may be different from thenumber illustrated in FIG. 9B, the MV predictors registered in the MVpredictor list may omit one or more of the types of MV predictors givenin the example in FIG. 9B, and the MV predictors registered in the MVpredictor list may include one or more types of MV predictors inaddition to and different from the types given in the example in FIG.9B.

Note that the final MV may be determined by performing DMVR processing(to be described later) by using the MV of the current block derived viamerge mode.

Here, an example of determining an MV by using DMVR processing will begiven.

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing.

First, the most appropriate MVP set for the current block is consideredto be the candidate MV, reference pixels are obtained from a firstreference picture, which is a picture processed in the L0 direction inaccordance with the candidate MV, and a second reference picture, whichis a picture processed in the L1 direction in accordance with thecandidate MV, and a template is generated by calculating the average ofthe reference pixels.

Next, using the template, the surrounding regions of the candidate MVsof the first and second reference pictures are searched, and the MV withthe lowest cost is determined to be the final MV. Note that the costvalue is calculated using, for example, the difference between eachpixel value in the template and each pixel value in the regionssearched, as well as the MV value.

Note that the outlines of the processes described here are fundamentallythe same in both the encoder and the decoder.

Note that processing other than the processing exactly as describedabove may be used, so long as the processing is capable of deriving thefinal MV by searching the surroundings of the candidate MV.

Here, an example of a mode that generates a prediction image by usingLIC processing will be given.

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing.

First, an MV is extracted for obtaining, from an encoded referencepicture, a reference image corresponding to the current block.

Next, information indicating how the luminance value changed between thereference picture and the current picture is extracted and a luminancecorrection parameter is calculated by using the luminance pixel valuesfor the encoded left neighboring reference region and the encoded upperneighboring reference region, and the luminance pixel value in the samelocation in the reference picture specified by the MV.

The prediction image for the current block is generated by performing aluminance correction process by using the luminance correction parameteron the reference image in the reference picture specified by the MV.

Note that the shape of the surrounding reference region illustrated inFIG. 9D is just one example; the surrounding reference region may have adifferent shape.

Moreover, although a prediction image is generated from a singlereference picture in this example, in cases in which a prediction imageis generated from a plurality of reference pictures as well, theprediction image is generated after performing a luminance correctionprocess, via the same method, on the reference images obtained from thereference pictures.

One example of a method for determining whether to implement LICprocessing is by using an lic_flag, which is a signal that indicateswhether to implement LIC processing. As one specific example, theencoder determines whether the current block belongs to a region ofluminance change. The encoder sets the lic_flag to a value of “1” whenthe block belongs to a region of luminance change and implements LICprocessing when encoding, and sets the lic_flag to a value of “0” whenthe block does not belong to a region of luminance change and encodeswithout implementing LIC processing. The decoder switches betweenimplementing LIC processing or not by decoding the lic_flag written inthe stream and performing the decoding in accordance with the flagvalue.

One example of a different method of determining whether to implementLIC processing is determining so in accordance with whether LICprocessing was determined to be implemented for a surrounding block. Inone specific example, when merge mode is used on the current block,whether LIC processing was applied in the encoding of the surroundingencoded block selected upon deriving the MV in the merge mode processingmay be determined, and whether to implement LIC processing or not can beswitched based on the result of the determination. Note that in thisexample, the same applies to the processing performed on the decoderside.

[Decoder Outline]

Next, a decoder capable of decoding an encoded signal (encodedbitstream) output from encoder 100 will be described. FIG. 10 is a blockdiagram illustrating a functional configuration of decoder 200 accordingto Embodiment 1. Decoder 200 is a moving picture/picture decoder thatdecodes a moving picture/picture block by block.

As illustrated in FIG. 10, decoder 200 includes entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, block memory210, loop filter 212, frame memory 214, intra predictor 216, interpredictor 218, and prediction controller 220.

Decoder 200 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, loop filter212, intra predictor 216, inter predictor 218, and prediction controller220. Alternatively, decoder 200 may be realized as one or more dedicatedelectronic circuits corresponding to entropy decoder 202, inversequantizer 204, inverse transformer 206, adder 208, loop filter 212,intra predictor 216, inter predictor 218, and prediction controller 220.

Hereinafter, each component included in decoder 200 will be described.

[Entropy Decoder]

Entropy decoder 202 entropy decodes an encoded bitstream. Morespecifically, for example, entropy decoder 202 arithmetic decodes anencoded bitstream into a binary signal. Entropy decoder 202 thendebinarizes the binary signal. With this, entropy decoder 202 outputsquantized coefficients of each block to inverse quantizer 204.

[Inverse Quantizer]

Inverse quantizer 204 inverse quantizes quantized coefficients of ablock to be decoded (hereinafter referred to as a current block), whichare inputs from entropy decoder 202. More specifically, inversequantizer 204 inverse quantizes quantized coefficients of the currentblock based on quantization parameters corresponding to the quantizedcoefficients. Inverse quantizer 204 then outputs the inverse quantizedcoefficients (i.e., transform coefficients) of the current block toinverse transformer 206.

[Inverse Transformer]

Inverse transformer 206 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 204.

For example, when information parsed from an encoded bitstream indicatesapplication of EMT or AMT (for example, when the AMT flag is set totrue), inverse transformer 206 inverse transforms the transformcoefficients of the current block based on information indicating theparsed transform type.

Moreover, for example, when information parsed from an encoded bitstreamindicates application of NSST, inverse transformer 206 applies asecondary inverse transform to the transform coefficients.

[Adder]

Adder 208 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 206, and prediction samples,which is an input from prediction controller 220. Adder 208 then outputsthe reconstructed block to block memory 210 and loop filter 212.

[Block Memory]

Block memory 210 is storage for storing blocks in a picture to bedecoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 210 storesreconstructed blocks output from adder 208.

[Loop Filter]

Loop filter 212 applies a loop filter to blocks reconstructed by adder208, and outputs the filtered reconstructed blocks to frame memory 214and, for example, a display device.

When information indicating the enabling or disabling of ALF parsed froman encoded bitstream indicates enabled, one filter from among aplurality of filters is selected based on direction and activity oflocal gradients, and the selected filter is applied to the reconstructedblock.

[Frame Memory]

Frame memory 214 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 214 stores reconstructed blocks filtered byloop filter 212.

[Intra Predictor]

Intra predictor 216 generates a prediction signal (intra predictionsignal) by intra prediction with reference to a block or blocks in thecurrent picture and stored in block memory 210. More specifically, intrapredictor 216 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 220.

Note that when an intra prediction mode in which a chroma block is intrapredicted from a luma block is selected, intra predictor 216 may predictthe chroma component of the current block based on the luma component ofthe current block.

Moreover, when information indicating the application of PDPC is parsedfrom an encoded bitstream, intra predictor 216 correctspost-intra-prediction pixel values based on horizontal/verticalreference pixel gradients.

[Inter Predictor]

Inter predictor 218 predicts the current block with reference to areference picture stored in frame memory 214. Inter prediction isperformed per current block or per sub-block (for example, per 4×4block) in the current block. For example, inter predictor 218 generatesan inter prediction signal of the current block or sub-block by motioncompensation by using motion information (for example, a motion vector)parsed from an encoded bitstream, and outputs the inter predictionsignal to prediction controller 220.

Note that when the information parsed from the encoded bitstreamindicates application of OBMC mode, inter predictor 218 generates theinter prediction signal using motion information for a neighboring blockin addition to motion information for the current block obtained frommotion estimation.

Moreover, when the information parsed from the encoded bitstreamindicates application of FRUC mode, inter predictor 218 derives motioninformation by performing motion estimation in accordance with thepattern matching method (bilateral matching or template matching) parsedfrom the encoded bitstream. Inter predictor 218 then performs motioncompensation using the derived motion information.

Moreover, when BIO mode is to be applied, inter predictor 218 derives amotion vector based on a model assuming uniform linear motion. Moreover,when the information parsed from the encoded bitstream indicates thataffine motion compensation prediction mode is to be applied, interpredictor 218 derives a motion vector of each sub-block based on motionvectors of neighboring blocks.

[Prediction Controller]

Prediction controller 220 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto adder 208.

[An Internal Structure of the Transformer in the Encoder]

Next, an example of an internal structure of transformer 106 in encoder100 is described with reference to FIG. 11.

FIG. 11 is a block diagram illustrating the internal structure oftransformer 106 in encoder 100 according to Embodiment 1. Transformer106 includes: size determiner 1061; basis selector 1062; and frequencytransformer 1063.

Size determiner 1061 determines whether or not a current block to beencoded has a size smaller than or equal to a threshold size. As thethreshold size indicating a border between block sizes for switchingbases, for example, a fixed size (such as 4×4 pixels) which has beendefined in the standardized standard. In addition, the threshold sizemay be determined based on an input image signal, or may be input froman external device or a user. For example, the threshold size may bedetermined based on an intra prediction mode, a quantization parameter,a prediction error, etc.

When the current block has a size larger than the threshold size, basisselector 1062 selects a basis for the current block from among aplurality of frequency transform bases. The basis is selected, forexample, based on (i) a prediction error, or (ii) an evaluation value(cost) determined with consideration of the prediction error and thecoding amount required for encoding the prediction error. For example,the basis that yields the smallest residual (prediction error) isselected from among a plurality of bases.

Information about the basis selected here is output to entropy encoder110 and inverse transformer 114. Entropy encoder 110 writes theinformation about the selected basis onto a bitstream. The informationabout the basis is information indicating the selected basis, andincludes, for example, values of respective elements of the selectedbasis. In addition, the information about the selected basis may be anindex indicating the selected basis. The information about the selectedbasis is written onto at least one of a plurality of headers indicatedin (i) to (v) of FIG. 12.

FIG. 12 illustrates examples of positions of selected basis informationin a bitstream according to Embodiment 1. Specifically, in FIG. 12, (i)indicates that information about a selected basis is present in a videoparameter set. In FIG. 12, (ii) indicates that the information about theselected basis is present in a sequence parameter set in a video stream.In FIG. 12, (iii) indicates that the information about the selectedbasis is present in a picture parameter set in a picture. In FIG. 12,(iv) indicates that the information about the selected basis is presentin a slice header in a slice. In FIG. 12, (v) indicates that theinformation about the selected basis is present in a parameter group forsetting or initializing a video system or a video decoder. When theinformation about the selected basis is present in each of a pluralityof hierarchical layers (for example, the picture parameter set and theslice header), the information about the selected basis present in alower layer (for example, the slice header) overwrites the informationabout the selected basis present in a higher layer (for example, thepicture parameter set).

A plurality of frequency transform bases which can be selected aredefined by the standardized standard, etc. in advance, and include, forexample, bases (basis functions) of DCT-II, DCT-V, DCT-VIII, DST-I, andDST-VII illustrated in FIG. 3. It is to be noted that the plurality offrequency transform bases are not limited to the bases illustrated inFIG. 3, and may include sixteen kinds of bases of DCT types I to III andDST types I to III. In addition, the plurality of frequency transformbases may include not only orthogonal transform bases but alsonon-orthogonal transform bases.

When the current block has a size smaller than or equal to the thresholdsize, frequency transformer 1063 transforms the current block using afixed frequency transform basis.

The fixed frequency transform basis is fixed irrespective of (i) aprediction error and (ii) an evaluation value determined withconsideration of the prediction error and the coding amount required forencoding the prediction error, and is, for example, determined by thestandardized standard, etc. in advance. More specifically, the fixedfrequency transform basis is a basis of, for example, DST-VII, DCT-V, orthe like. It is to be noted that the fixed frequency transform basis maybe determined adaptively according to a residual of the current blockand one of an intra prediction mode and a quantization parameter, etc.In this case, information about the fixed frequency transform basis maybe written onto a bitstream.

In addition, when the current block has a size larger than the thresholdsize, frequency transformer 1063 transforms the current block using thebasis selected by basis selector 1062.

It is to be noted that a coefficient of the current block output fromfrequency transformer 1063 is quantized and inverse quantized byquantizer 108 and inverse quantizer 112, respectively. Inversetransformer 114 performs inverse frequency transform on the coefficientof the current block which has been quantized and inverse quantized. Atthis time, inverse transformer 114 inverse transforms the current blockusing an inverse frequency transform basis corresponding to thefrequency transform basis used by frequency transformer 1063.

[Operations Performed by the Transformer of the Encoder]

Next, operations performed by transformer 106 configured as describedabove are described specifically with reference to FIG. 13. FIG. 13 is aflowchart indicating operations performed by transformer 106 in encoder100 according to Embodiment 1.

First, size determiner 1061 determines whether or not a current block tobe encoded has a size smaller than or equal to a threshold size (S101).Here, when the current block has a size smaller than or equal to athreshold size (Yes in S101), frequency transformer 1063 transforms thecurrent block using a fixed frequency transform basis (S102). Forexample, frequency transformer 1063 transforms the current block havinga 4×4 size using a basis of DST-VII.

When the current block has a size larger than the threshold size (No inS101), basis selector 1062 selects a basis for the current block fromamong a plurality of frequency transform bases (S103). For example,basis selector 1062 selects one basis from among bases of type I to typeVIII, based on an evaluation value (cost) determined with considerationof the coding amount. Frequency transformer 1063 then transforms thecurrent block using the selected basis (S104).

[An Internal Structure of a Transformer in a Decoder]

Next, a description is given of an internal structure of inversetransformer 206 of decoder 200.

FIG. 14 is a block diagram illustrating an internal structure of inversetransformer 206 in decoder 200 according to Embodiment 1. Inversetransformer 206 includes: size determiner 2061; basis obtainer 2062; andinverse frequency transformer 2063.

Size determiner 2061 determines whether or not a current block to bedecoded has a size smaller than or equal to a threshold size. Sizedeterminer 2061 makes the determination, for example, based oninformation about the size of the current block which is obtainable froma bitstream.

When the current block has a size larger than the threshold size, basisobtainer 2062 obtains a basis for the current block based on theinformation about the selected basis included in the bitstream. Theinformation about the selected basis is information for identifying aninverse frequency transform basis corresponding to the frequencytransform basis selected by basis selector 1062 of encoder 100. In otherwords, basis obtainer 2062 obtains the inverse frequency transform basiscorresponding to the frequency transform basis selected by basisselector 1062 of encoder 100.

When the current block has the size larger than the threshold value,inverse frequency transformer 2063 performs inverse frequency transformon the current block using the basis obtained by basis obtainer 2062. Inaddition, when the current block has the size smaller than or equal tothe threshold size, inverse frequency transformer 2063 inversetransforms the current block using a fixed inverse frequency transformbasis.

The fixed inverse frequency transform basis is fixed irrespective of (i)a prediction error and (ii) an evaluation value determined withconsideration of the prediction error and the coding amount required forencoding the prediction error, and is, for example, determined by thestandardized standard, etc. in advance. It is to be noted thatinformation about the fixed inverse frequency transform basis may beparsed from the bitstream.

[Operations Performed by the Inverse Transformer of the Decoder]

Next, operations performed by inverse transformer 206 configured asdescribed above are described specifically with reference to FIG. 15.FIG. 15 is a flowchart indicating operations performed by inversetransformer 206 in decoder 200 according to Embodiment 1.

First, size determiner 2061 determines whether or not a current block tobe decoded has a size smaller than or equal to a threshold size (S201).Here, when the current block has a size smaller than or equal to thethreshold size (Yes in S201), inverse frequency transformer 2063 inversetransforms the current block using a fixed inverse frequency transformbasis (S202). When the current block has a size larger than thethreshold size (No in S201), basis obtainer 2062 obtains a basis for thecurrent block based on the information about the selected basis includedin a bitstream (S202). Inverse frequency transformer 2063 then inversetransforms the current block using the obtained basis (S204).

[Effects, Etc.]

As described above, transformer 106 of encoder 100 and inversetransformer 206 of decoder 200 according to this embodiment are capableof transforming and inverse transforming the current block to be encodedand the current block to be decoded using the fixed frequency transformbasis and fixed inverse frequency transform basis, respectively. In thiscase, cost evaluation, etc. for selecting a basis is unnecessary, whichreduces the load and time for encoding. When the current blocks have thesize larger than the threshold size, transformer 106 and inversetransformer 206 are capable of transforming and inverse transforming thecurrent blocks using the basis selected from among the plurality offrequency transform bases and inverse transform bases corresponding tothe selected bases. In this case, the basis suitable for the currentblock can be used, and thus the compression efficiency can be increased.In this way, by switching the fixed basis and the selected basisaccording to the size of each current block, it is possible to reduceincrease in the load or time for encoding while increasing thecompression efficiency.

In addition, with the use of transformer 106 of encoder 100 and inversetransformer 206 of decoder 200 according to this embodiment, it ispossible to include the information about the selected basis onto thebitstream when the current blocks have the size larger than thethreshold size. Thus, the decoder can perform inverse frequencytransform using an appropriate basis. Furthermore, when the currentblocks have the size smaller than or equal to the threshold size, thereis no need to include information about a basis in the bitstream. Inother words, information about a basis needs to be included in thebitstream only when the current blocks have the size larger than thethreshold size. Thus, the coding amount for the information about thebasis can be reduced, and the compression efficiency can be increased.

Variation 1 of Embodiment 1

Next, Variation 1 of Embodiment 1 is described. This variation differsfrom the above embodiment in that information about a threshold size isincluded in a bitstream. Hereinafter, this variation is describedspecifically focusing on the differences from Embodiment 1 withreference to FIGS. 16 to 19.

[An Internal Structure of a Transformer in an Encoder]

FIG. 16 is a block diagram illustrating an internal structure oftransformer 106A in encoder 100 according to Variation 1 ofEmbodiment 1. Transformer 106A includes: size determiner 1061; basisselector 1062; frequency transformer 1063; and threshold size determiner1064A.

Threshold size determiner 1064A determines a threshold size adaptivelyaccording to an input image signal, etc. The determined threshold sizeis used by size determiner 1061.

In addition, information about the determined threshold size is outputto entropy encoder 110, and is written onto the bitstream. Theinformation about the threshold size is information for identifying thethreshold size, and is, for example, a value indicating the thresholdsize itself. Alternatively, the information about the threshold size maybe an index indicating the threshold size. The information about thethreshold size is, for example, written onto at least one of a pluralityof headers indicated in (i) to (v) in FIG. 12, similarly to the case ofthe information about the selected basis. It is to be noted that thethreshold size information does not always need to be written in theheader in which the selected basis information and the threshold sizeinformation are written, and may be written in a different header.

[Operations Performed by the Transformer of the Encoder]

Next, operations performed by inverse transformer 106A according to thisvariation configured as described above are described specifically withreference to FIG. 17. FIG. 17 is a flowchart indicating operationsperformed by transformer 106A in encoder 100 according to Variation 1 ofEmbodiment 1.

First, threshold size determiner 1064A adaptively determines a thresholdsize, and outputs information about the determined threshold size toentropy encoder 110 (S111). Subsequently, processing in Step S101 andthe subsequent steps is executed as in Embodiment 1.

[An Internal Structure of an Inverse Transformer in a Decoder]

Next, a description is given of an internal structure of inversetransformer 206A of decoder 200. FIG. 18 is a block diagram illustratingan internal structure of inverse transformer 206A in decoder 200according to Variation 1 of Embodiment 1. Inverse transformer 206Aincludes: size determiner 2061; basis obtainer 2062; inverse frequencytransformer 2063; and threshold size obtainer 2064A.

Threshold size obtainer 2064A obtains the threshold size from thebitstream. For example, threshold size obtainer 2064A obtains thethreshold size based on the information about the threshold size parsedfrom the bitstream. The obtained threshold size is used by sizedeterminer 2061.

[Operations Performed by the Inverse Transformer of the Decoder]

Next, operations performed by inverse transformer 206A according to thisvariation configured as described above are described specifically withreference to FIG. 19. FIG. 19 is a flowchart indicating operationsperformed by inverse transformer 206A in decoder 200 according toVariation 1 of Embodiment 1.

First, threshold size obtainer 2064A obtains a threshold size from abitstream (S211). Subsequently, processing in Step S201 and thesubsequent steps is executed as in Embodiment 1.

[Effects, Etc.]

As described above, with the use of transformer 106A of encoder 100 andinverse transformer 206A of decoder 200 according to this variation, itis possible to include the information about the threshold size in thebitstream. Accordingly, the threshold size can be determined adaptivelyaccording to an input image, and compression efficiency can be furtherincreased.

Variation 2 of Embodiment 1

Next, Variation 1 of Embodiment 1 is described. This variation differsfrom Variation 1 of Embodiment 1 in a frequency transform basisselecting method in the case where a current block has a size largerthan a threshold size. Hereinafter, this variation is describedspecifically focusing on the differences from Variation 1 of Embodiment1 with reference to FIGS. 20 to 22.

[An Internal Structure of a Transformer in an Encoder]

FIG. 20 is a block diagram illustrating an internal structure oftransformer 106B in encoder 100 according to Variation 2 ofEmbodiment 1. Transformer 106B includes: size determiner 1061; basisselector 1062B; frequency transformer 1063; and threshold sizedeterminer 1064A.

Basis selector 1062B selects one basis set from a plurality of basissets based on a predetermined condition. In other words, basis selector1062B determines whether the current block satisfies the predeterminedcondition, and selects the basis set based on the result ofdetermination.

Each of the plurality of basis sets includes an arbitrary combination ofa plurality of frequency transform bases. Here, the number of basesincluded in each of the plurality of basis sets is fewer than the numberof frequency transform bases which can be selected in one of Embodiment1 and Variation 1 thereof. In other words, the number of bases includedin each basis set is limited. In addition, the basis set does not alwaysneed to include a plurality of bases, and may include only one basis.

The predetermined condition is defined by information which can beobtained without requiring cost evaluation of the current block. Forexample, the predetermined condition is defined according to an intraprediction mode for the current block. In addition, the predeterminedcondition may be defined by a random number, or may be defined by apredetermined probability for selecting each basis.

When the predetermined condition is defined by the intra predictionmode, basis selector 1062B selects a basis set, for example, in thefollowing manner. When the intra prediction mode for the current blockis a first intra prediction mode, basis selector 1062B selects a firstbasis set corresponding to the first intra prediction mode. When theintra prediction mode for the current block is a second intra predictionmode, basis selector 1062B selects a second basis set corresponding tothe second intra prediction mode. Here, the first intra prediction modeand the second intra prediction mode are different from each other, andthe first basis set and the second basis set are also different fromeach other.

Furthermore, basis selector 1062B selects a basis for the current blockfrom the selected basis set. The basis is selected, for example, basedon (i) a prediction error, or (ii) an evaluation value (cost) determinedwith consideration of the prediction error and the coding amountrequired for encoding the prediction error. For example, the basis thatyields the smallest residual (prediction error) is selected from among aplurality of bases.

The information about the basis selected here is output to entropyencoder 110 and inverse transformer 114, and is written onto thebitstream.

[Operations Performed by the Transformer of the Encoder]

Next, operations performed by transformer 106B configured as describedabove are described specifically with reference to FIGS. 21 and 22. FIG.21 is a flowchart indicating operations performed by transformer 106B inencoder 100 according to Variation 2 of Embodiment 1.

When a current block has a size larger than a threshold size (No inS101), basis selector 1062B selects a basis for the current block fromamong a plurality of frequency transform bases (S121).

Here, a description is given of details of basis selection in Step S121with reference to FIG. 22. FIG. 22 is a flowchart indicating operationsperformed by basis selector 1062B in encoder 100 according to Variation2 of Embodiment 1.

Basis selector 1062B determines whether the current block satisfies afirst condition (S1211). Specifically, basis selector 1062B determines,for example, whether the value of the intra prediction mode for thecurrent block is a predetermined first value.

Here, when the current block satisfies the first condition (Yes inS1211), basis selector 1062B selects a first basis set (S1212). When thecurrent block does not satisfy the first condition (No in S1211), basisselector 1062B determines whether the current block satisfies a secondcondition (S1213). Specifically, basis selector 1062B determines, forexample, whether the value of the intra prediction mode for the currentblock is a predetermined second value.

Here, when the current block satisfies the second condition (Yes inS1213), basis selector 1062B selects a second basis set (S1214). Whenthe current block does not satisfy the second condition (No in S1213),basis selector 1062B determines whether the current block satisfies ani-th condition (2<i<N, i and N are each a natural number). When thecurrent block satisfies the i-th condition, an i-th basis set isselected. When the current block does not satisfy the (N−1)-thcondition, basis selector 1062B selects an N-th basis set (S1215). Inthis way, any one of the first to N-th basis sets is selected.

Basis selector 1062B then selects a basis for the current block fromamong the selected basis set (S1216). In other words, basis selector1062B selects the basis for the current block from the at least onebasis included in the basis set. The number of bases here is smallerthan the number of bases which can be selected in Embodiment 1 andVariation 1 thereof.

[Effects, Etc.]

As described above, transformer 106B of encoder 100 according to thisvariation is capable of selecting, based on the predetermined condition,the basis for the current block included in the basis set selected fromamong the plurality of basis sets. Accordingly, selectable bases can belimited according to the predetermined condition, and the load and timefor encoding can be reduced.

In addition, transformer 106B of encoder 100 according to this variationis capable of selecting the basis set based on the intra prediction modefor the current block. The intra prediction mode corresponds to an intraprediction direction, and thus affects a residual distribution in thecurrent block. Accordingly, by selecting a basis set based on the intraprediction mode, the basis set including a limited number of basessuitable for the residual distribution in the current block can beselected, and thus efficient basis selection and increase in compressionefficiency can be achieved.

Variation 3 of Embodiment 1

Next, Variation 1 of Embodiment 3 is described. This variation differsfrom Variation 2 of Embodiment 2 in that it is possible to switch afirst mode for use in the transform and inverse transform according toVariation 2 of Embodiment 1 and a second mode for other transform andinverse transform. Hereinafter, this variation is described specificallyfocusing on the differences from Variation 2 of Embodiment 1 withreference to FIGS. 23 to 28.

[An Internal Structure of a Transformer in an Encoder]

FIG. 23 is a block diagram illustrating an internal structure oftransformer 106C in encoder 100 according to Variation 3 ofEmbodiment 1. Transformer 106C includes: size determiner 1061; basisselector 1062B; frequency transformer 1063; threshold size determiner1064A; and transform mode determiner 1065C.

Transform mode determiner 1065C determines which one of the plurality oftransform modes including the first transform mode and the secondtransform mode is to be applied to a current block. The plurality oftransform modes may be different from each other in, for example,selectable bases, or in method for selecting the same selectable basis.

Information about the transform mode to be applied to the current blockis output to entropy encoder 110, and is written onto a bitstream. Theinformation about a transform mode is information for identifying thetransform mode, and is, for example, a flag or an index indicating thetransform mode. The information about the transform mode is, forexample, written onto at least one of a plurality of headers indicatedin (i) to (v) in FIG. 12, similarly to the case of the information aboutthe selected basis and the information about the threshold size. It isto be noted that the information about the transform mode does notalways need to be written in the header in which the information aboutthe selected basis and the information about the threshold size arewritten, and may be written in a different header.

When the first transform mode is applied and the current block has asize larger than the threshold size, basis selector 1062B selects abasis using a selecting method similar to the selecting method inVariation 2 of Embodiment 1. Frequency transformer 1063C then transformsthe current block using the basis selected by basis selector 1062B.

When the first transform mode is applied and the current block has asize smaller than or equal to the threshold size, frequency transformer1063C transforms the current block using a first fixed basis for thefirst transform mode.

In this variation, the frequency transform in the first transform modeas such is referred to as first frequency transform.

When the second transform mode is applied and the current block has asize smaller than or equal to the threshold size, frequency transformer1063C transforms the current block using a second fixed basis for thesecond transform mode. When the second transform mode is applied and thecurrent block has a size larger than the threshold size, frequencytransformer 1063C transforms the current block using a third fixed basisfor the second transform mode. In this variation, the frequencytransform in the second transform mode as such is referred to as secondfrequency transform.

The first frequency transform is the same as the frequency transformaccording to Variation 2 of Embodiment 1. The second frequency transformdiffers from the first frequency transform. Here, the second frequencytransform uses a fixed basis even when a current block has a size largerthan the threshold value.

[Operations Performed by the Transformer of the Encoder]

Next, operations performed by inverse transformer 106C according to thisvariation configured as described above are described specifically withreference to FIGS. 24 and 25. FIGS. 24 and 25 are each a flowchartindicating processing performed by transformer 106C of encoder 100according to Variation 3 of Embodiment 1.

A threshold size is determined and output (S111) first, and thentransform mode determiner 1065C determines a transform mode to beapplied to a current block, and outputs the transform mode to entropyencoder 110 (S131).

Here, when the determined transform mode is a first transform mode (thefirst transform mode in S131), processing in Step S101 and thesubsequent steps is executed. When the determined transform mode is asecond transform mode (the second transform mode in S131), processing inSteps S132 to S134 in FIG. 25 is executed.

Specifically, size determiner 1061 determines whether or not the currentblock has a size smaller than or equal to the threshold size (S132).Here, when the current block has a size smaller than or equal to thethreshold size (Yes in S132), frequency transformer 1063C transforms thecurrent block using the second fixed basis for the second transform mode(S133). When the current block has a size larger than the thresholdvalue (No in S132), frequency transformer 1063C transforms the currentblock using the third fixed basis for the second transform mode.

As the fixed basis, it is possible to use any one of eight kinds ofbases of type I to type III defined based on a border condition orsymmetry in each of DCT and DST. For example, it is possible to use abasis of DST-VII as the first fixed basis for the first transform mode,a basis of DCT-V as the second fixed basis for the second transformmode, and a basis of DCT-II as the third fixed basis for the secondtransform mode. It is to be noted that the second or third fixed basisfor the second transform mode may be the same as the first fixed basisfor the first transform mode.

[An Internal Structure of an Inverse Transformer in a Decoder]

Next, a description is given of an internal structure of inversetransformer 206C of decoder 200. FIG. 26 is a block diagram illustratingan internal structure of inverse transformer 206C in decoder 200according to Variation 3 of Embodiment 1. Inverse transformer 206Cincludes: size determiner 2061; basis obtainer 2062; inverse frequencytransformer 2063C; threshold size obtainer 2064A; and transform modedeterminer 2065C.

Transform mode determiner 2065C determines which one of the plurality oftransform modes including the first transform mode and the secondtransform mode is to be applied to the current block. For example,transform mode determiner 2065C determines a transform mode based oninformation about the determined transform mode parsed from thebitstream by entropy decoder 202.

When the first transform mode is applied and the current block has asize smaller than or equal to the threshold size, inverse frequencytransformer 2063C inverse transforms the current block using the firstfixed basis for the first transform mode. When the first transform modeis applied and the current block has a size smaller than or equal to thethreshold size, inverse frequency transformer 2063C inverse transformsthe current block using a basis obtained by basis obtainer 2062. In thisvariation, the frequency transform in the first transform mode as suchis referred to as first inverse frequency transform.

When the second transform mode is applied and the current block has asize smaller than or equal to the threshold size, inverse frequencytransformer 2063C inverse transforms the current block using the secondfixed basis for the second transform mode. When the second transformmode is applied and the current block has a size smaller than or equalto the threshold size, inverse frequency transformer 2063C inversetransforms the current block using the third fixed basis for the thirdtransform mode. In this variation, the inverse frequency transform inthe second transform mode as such is referred to as second inversefrequency transform.

[Operations Performed by the Inverse Transformer of the Decoder]

Next, operations performed by inverse transformer 206C according to thisvariation configured as described above are described specifically withreference to FIGS. 27 and 28. FIGS. 27 and 28 are each a flowchartindicating processing performed by transformer 206C of decoder 200according to Variation 3 of Embodiment 1.

First, threshold size obtainer 2064A obtains a threshold size from abitstream (S211). Subsequently, transform mode determiner 2065Cdetermines which one of a plurality of transform modes is to be appliedto the current block (S231). Here, when the first transform mode isapplied (the first transform mode in S231), processing in Step S201 andthe subsequent steps is executed. When the second transform mode isapplied (the second transform mode in S231), size determiner 2061determines whether or not the current block has a size smaller than orequal to the threshold size (S232).

Here, when the current block has a size smaller than or equal to thethreshold size (Yes in S232), inverse frequency transformer 2063Cinverse transforms the current block using the second fixed basis forthe second transform mode (S233). When the current block has a sizelarger than the threshold size (No in S232), inverse frequencytransformer 2063C inverse transforms the current block using the thirdfixed basis for the second transform mode (S234).

[Effects, Etc.]

As described above, transformer 106C of encoder 100 and inversetransformer 206C of decoder 200 according to this variation are capableof switching the plurality of frequency transforms using transformmodes. Accordingly, efficiency of frequency transform can be furtherincreased, which enables further increase in compression efficiency.

Furthermore, with the use of transformer 106 of encoder 100 and inversetransformer 206C of decoder 200 according to this variation, it ispossible to include information about a transform mode to be applied tothe current block in the bitstream. Accordingly, a transform mode can bedetermined adaptively according to an input image, which enables furtherincrease in compression efficiency.

It is to be noted that, although this variation is described focusing onthe cases in which the two transform modes (the first transform mode andthe second transform mode) are used, the number of transform modes isnot limited to two. For example, in addition to the first transform modeand the second transform mode, a third transform mode and/or a fourthtransform mode may be used.

Variation 4 of Embodiment 1

Next, Variation 1 of Embodiment 4 is described. This variation differsfrom Embodiment 1 in that transform basis candidates/inverse transformbasis candidates are determined according to the size of a currentblock, and that a basis for the current block is selected from thedetermined transform basis candidates. This variation is described belowfocusing on differences from Embodiment 1. It is to be noted that theconfigurations of encoder 100 and decoder 200 according to thisvariation are identical or similar to those in Embodiment 1, and thusare not illustrated in the drawings and not described here.

[Operations Performed by the Transformer of the Encoder]

First, operations performed by transformer 106 of encoder 100 accordingto this variation are specifically described with reference to FIG. 29.FIG. 29 is a flowchart indicating the operations performed bytransformer 106 of encoder 100 according to Variation 4 of Embodiment 1.

First, transformer 106 selects a plurality of transform basis candidates(S141). For example, transformer 106 may select a plurality of transformbasis candidates which have been defined in advance in a standard, orthe like. In addition, for example, transformer 106 may adaptivelyselect a plurality of transform basis candidates. The plurality oftransform basis candidates selected here corresponds to a plurality offirst transform basis candidates.

After the plurality of first transform basis candidates is selected,transformer 106 determines whether the block size of the current blocksatisfies a predetermined condition (S142). The predetermined conditionhere means a condition regarding a predetermined block size. Thepredetermined condition may be defined in advance in a standard, etc.,or may be adaptively determined based on a cost etc. Specifically, forexample, the predetermined condition indicates that the block size islarger than or equal to and/or smaller than or equal to a thresholdsize. In addition, for example, the predetermined condition may indicatethat the block size is the same or different from a predetermined size(for example, 16×16). In addition, the predetermined condition may be acondition in which these conditions are combined.

Here, in the case where the block size of the current block satisfiesthe predetermined condition (Yes in S142), transformer 106 may reducethe number of the plurality of transform basis candidates selected inStep S141 (S143). In other words, transformer 106 excludes at least onetransform basis from the selected plurality of transform basiscandidates. At this time, the reduced number of transform basiscandidates may be 1 or may be 2 or more. The transform basiscandidate(s) reduced in this way correspond(s) to one or more secondtransform basis candidates. Transformer 106 selects a transform basisfor a current block to be encoded from the transform basis candidates(that are the one or more second transform basis candidates) reduced inStep S143 (S144). Specifically, transformer 106 selects a transformbasis based on an evaluation value (cost) with consideration of aprediction error, or a prediction error and the coding amount of theprediction error.

Transformer 106 transforms a current block to be encoded using thetransform basis selected in Step S144 (S145), and ends the processing.Specifically, for example, transformer 106 performs frequency transformon residuals of a block to be encoded using the selected transform basisto generate frequency coefficients.

In the opposite case where the block size of the current block does notsatisfy the predetermined condition (No in S142), transformer 106selects a transform basis for the current block from the plurality oftransform basis candidates (that are the plurality of first transformbasis candidates) selected in Step S141 (S146). Specifically,transformer 106 selects a transform basis based on an evaluation value(cost) with consideration of a prediction error, or a prediction errorand the coding amount of the prediction error.

Transformer 106 transforms the current block using the transform basisselected in Step S146 (S147), and ends the processing. Specifically, forexample, transformer 106 performs frequency transform on residuals of ablock to be encoded using the selected transform basis to generatefrequency coefficients.

It is to be noted that, in Step S143, the at least one transform basisexcluded from the plurality of transform basis candidates may bedetermined, for example, based on the possibility of fast computation.In other words, transformer 106 may exclude, from the transform basiscandidates, the at least one transform basis which does not enable fastcomputation (which does not allow use of any fast computation method)for a block having a size that satisfies a predetermined condition. Fastcomputation is computation which requires a smaller processing loadand/or a shorter processing time than normal computation. Specifically,fast computation may be butterfly computation. In addition, fastcomputation may be computation which requires a computation amountsmaller than or equal to a predetermined amount.

For example, when the predetermined condition used is that a block has ablock size larger than a threshold size, transformer 106 determines aplurality of first transform basis candidates in the case where acurrent block to be encoded has a first size smaller than the thresholdsize. In the opposite case where the current block to be encoded has asecond size larger than the threshold size, transformer 106 determinesone or more second transform basis candidates. At this time, the numberof the one or more second transform basis candidates is smaller than thenumber of the plurality of first transform basis candidates.Furthermore, each of the one or more second transform basis candidatesis included in the plurality of first transform basis candidates. Inother words, the one or more second transform basis candidates is a truesubset of the plurality of first transform basis candidates.

[Examples of Transform Bases which Enable Fast Computation]

Here, examples of transform bases which enable fast computation forblock sizes are described with reference to FIG. 30. FIG. 30 is adiagram indicating the examples of the transform bases which enable fastcomputation for the block sizes. In FIG. 30, the following bases areemployed as transform bases: DCT-II (DCT2), DCT-V (DCT5), DCT-VIII(DCT8), DST-I (DST1), and DST-VII (DST7).

The circles in the diagram indicate the possibility of butterflycomputation, that is, the circles indicate that fast computation ispossible. The crosses in the diagram indicate the impossibility ofbutterfly computation, that is, the circles indicate that fastcomputation is impossible.

In FIG. 30, for example, all of DCT2, DCT5, DCT8, DST1, and DST7 enablefast computation for Size 4 and Size 8. Accordingly, when a currentblock to be encoded does not have a block size of at least 16×16 pixels,transform bases DCT2, DCT5, DCT8, DST1, and DST7 can be used as firsttransform basis candidates.

For example, in Size 16, DCT2, DCT8, DST1, and DST7 enable fastcomputation, but DCT5 does not enable fast computation. Accordingly,when a current block has a block size of 16×16 pixels, the transformbasis DCT5 may be excluded from the plurality of transform basiscandidates. In other words, when the block size is 16×16 pixels, DCT2,DCT8, DST1, and DST7 can be used as second transform basis candidates.

For example, in Size 32, DCT2 and DCT5 enable fast computation, butDCT8, DST1, and DCT7 do not enable fast computation. Accordingly, when acurrent block has a block size of 32×32 pixels, the transform basesDCT8, DCT1, and DST7 may be excluded from the plurality of transformbasis candidates. In other words, when the block size is 32×32 pixels,DCT2 and DCT5 can be used as second transform basis candidates.

It is to be noted that the number of bits to be used for signalling forthe selected transform bases may be changed when the transform basiscandidates are reduced. For example, when the number of transform basiscandidates is changed from 4 to 1, it is also possible to skipsignalling regarding information about a transform basis to be appliedto a current block (for example, information indicating whether EMT orAMT is to be applied and/or information indicating a transform basis).In addition, for example, when the number of transform basis candidatesis changed from 4 to 2, not 2-bit signal but 1-bit flag may be encodedfor information about a transform basis to be applied to a currentblock.

It is to be noted that a decoder side also needs to perform, forexample, a process for making a determination regarding the block sizeof a current block to be decoded, and calculate how many bits arerequired in the signalling.

[Operations Performed by the Inverse Transformer of the Decoder]

Next, operations performed by inverse transformer 206 of decoder 200according to this variation are specifically described with reference toFIG. 31. FIG. 31 is a flowchart indicating the operations performed byinverse transformer 206 of decoder 200 according to Variation 4 ofEmbodiment 1.

First, inverse transformer 206 selects a plurality of inverse transformbasis candidates (S241). Inverse transformer 206 selects a plurality ofinverse transform basis candidates corresponding to the plurality oftransform basis candidates selected by transformer 106 of encoder 100.The plurality of inverse transform basis candidates selected herecorresponds to a plurality of first inverse transform basis candidates.

Inverse transformer 206 determines whether a current block to be decodedhas a block size that satisfies a predetermined condition (S242). Thesame predetermined condition as the predetermined condition used inencoder 100 is used as the predetermined condition.

Here, in the case where the block size of the current block satisfiesthe predetermined condition (Yes in S242), inverse transformer 206reduces the number of the plurality of inverse transform basiscandidates selected in Step S241 (S243). In other words, inversetransformer 206 excludes one or more inverse transform basescorresponding to the one or more transform bases excluded in encoder 100from the selected plurality of inverse transform bases. The inversetransform basis candidate(s) reduced in this way correspond(s) to one ormore second inverse transform basis candidates.

Inverse transformer 206 selects inverse transform bases for the currentblock from the inverse transform basis candidates (that are the one ormore second inverse transform basis candidates) reduced in Step S243(S244). Specifically, inverse transform 206 selects the inversetransform basis based on information about the transform basis obtainedfrom a bitstream.

Inverse transformer 206 inverse-transforms the current block using theinverse transform basis selected in Step S244 (S245), and ends theprocessing. Specifically, for example, inverse transformer 206 performsinverse frequency transform on frequency coefficients of the currentblock using the selected inverse transform basis to generate residualsof the current block.

In the opposite case where the block size of the current block does notsatisfy the predetermined condition (No in S242), inverse transformer206 selects an inverse transform basis for the current block from theplurality of inverse transform basis candidates (that are the pluralityof first inverse transform basis candidates) selected in Step S241(S246). Specifically, inverse transform 206 selects the inversetransform basis based on information about the transform basis obtainedfrom a bitstream.

Inverse transformer 206 inverse-transforms the current block using theinverse transform basis selected in Step S246 (S247), and ends theprocessing. Specifically, for example, inverse transformer 206 performsinverse frequency transform on frequency coefficients of the currentblock using the selected inverse transform basis to generate residualsof the current block.

For example, in the case where the predetermined condition used is thata block has a block size larger than a threshold size, inversetransformer 206 determines a plurality of first inverse transform basiscandidates when a current block to be decoded has a first size smallerthan the threshold size. In the opposite case where the current block tobe decoded has a second size larger than the threshold size, inversetransformer 206 determines one or more second inverse transform basiscandidates. At this time, the number of the one or more second inversetransform basis candidates is smaller than the number of the pluralityof first inverse transform basis candidates. In addition, each of theone or more second inverse transform basis candidates is included in theplurality of first inverse transform basis candidates. In other words,the one or more second inverse transform basis candidates is a truesubset of the plurality of first inverse transform basis candidates.

[Effects, Etc.]

As described above, with encoder 100 and decoder 200 according to thisvariation, when the block size of the current block satisfies thepredetermined condition, it is possible to reduce the transform basiscandidates, and thus to reduce the cost for signalling transform basisinformation and reduce the number of transform coefficients byadaptively selecting the transform basis from the plurality of transformbasis candidates. Furthermore, it is also possible to exclude the one ormore transform bases from the transform basis candidates based on thecomputation amounts, and thus to reduce processing load and/orprocessing time.

In particular, when the current block to be encoded has the large blocksize, it is possible to reduce the processing load and/or processingtime more effectively by excluding the one or more transform bases whichrequire the large computation amount from the transform basiscandidates. In addition, it is also possible to reduce the circuit scaleof a dedicated circuit by excluding the one or more transform baseswhich require the large computation amount from the transform basiscandidates.

Variation 5 of Embodiment 1

Next, Variation 5 of Embodiment 1 is described. This variation differsfrom Variation 4 in that transform basis candidates are determined foreach of conditions regarding block sizes, instead of reducing the numberof transform basis candidates according to the result of a determinationmade after a plurality of transform basis candidates are selected inadvance. This variation is described below focusing on differences fromVariation 4 of Embodiment 1.

[Operations Performed by the Transformer of the Encoder]

First, operations performed by transformer 106 of encoder 100 accordingto this variation are specifically described with reference to FIG. 32.FIG. 32 is a flowchart indicating the operations performed bytransformer 106 of encoder 100 according to Variation 5 of Embodiment 1.

In this variation, as illustrated in FIG. 32, transformer 106 determineswhether the block size of a current block to be encoded satisfies apredetermined condition (S142). Here, in the case where the block sizesatisfies the predetermined condition (Yes in S142), transformer 106determines a plurality of first transform basis candidates (S151). Forexample, DCT2, DST5, and DCT8 can be used as the plurality of firsttransform basis candidates.

Subsequently, transformer 106 selects a transform basis for the currentblock from the determined first transform basis candidates (S152).Transformer 106 then transforms the current block using the transformbasis selected in Step S152 (S153), and ends the processing.

In the opposite case where the block size does not satisfy thepredetermined condition (No in S142), transformer 106 determines one ormore second transform basis candidates (S154). The one or more secondtransform basis candidates are different from the plurality of firsttransform basis candidates. Here, “different” means being not completelymatching. In other words, the plurality of first transform basiscandidates and the one or more second transform basis candidates mayinclude one or more common transform bases.

For example, transform bases DST1 and DST7 can be used as the one ormore second transform basis candidates. Subsequently, transformer 106selects a transform basis for the current block from the determined oneor more second transform basis candidates (S155). Transformer 106 thentransforms the current block using the transform basis selected in StepS155 (S156), and ends the processing.

Although the number of conditional branches in FIG. 32 is 2, it is to benoted that the number of the conditional branches is not limited to 2.For example, the number of conditional branches may be 3 or more. Inaddition, the number of transform basis candidates may be different orthe same for each condition.

[Operations Performed by the Inverse Transformer of the Decoder]

Next, operations performed by inverse transformer 206 of decoder 200according to this variation are specifically described with reference toFIG. 33. FIG. 33 is a flowchart indicating the operations performed byinverse transformer 206 of decoder 200 according to Variation 5 ofEmbodiment 1.

In this variation, as illustrated in FIG. 33, inverse transformer 206determines whether a current block to be decoded has a block size thatsatisfies a predetermined condition (S242). Here, in the case where theblock size satisfies a predetermined condition (Yes in S242), inversetransformer 206 determines a plurality of first inverse transform basiscandidates (S251). For example, inverse transform bases DCT2, DST5, andDCT8 are determined as first inverse transform basis candidates.Subsequently, inverse transformer 206 selects an inverse transform basisfor the current block from the determined first inverse transform basiscandidates (S252). Inverse transformer 206 then inverse-transforms thecurrent block using the inverse transform basis selected in Step S252(S253), and ends the processing.

In the opposite case where the block size does not satisfy thepredetermined condition (No in S242), inverse transformer 206 determinesone or more second inverse transform basis candidates (S254). Forexample, inverse transform bases DST1 and DST7 are determined as secondinverse transform basis candidates. Subsequently, inverse transformer206 selects an inverse transform basis for the current block from thedetermined second inverse transform basis candidates (S255). InverseTransformer 206 then inverse transforms the current block using theinverse transform basis selected in Step S255 (S256), and ends theprocessing.

Effects, Etc., of Variation 4 of Embodiment 1

In Variation 4 of Embodiment 1, the one or more second transform basiscandidates are obtained by excluding some transform bases from theplurality of first transform basis candidates. As a result, the secondtransform basis candidates depend on the first transform basiscandidates and are included in first transform bases. In this variation,however, it is possible to remove the condition that the first transformbasis candidates and the second transform basis candidates are in aninclusive relationship. Accordingly, it is possible to flexiblydetermine the first and second transform basis candidates, therebyincreasing coding efficiency. For example, it is possible toindependently determine transform basis candidates (first transformbasis candidates) in the case where the size of a current block to beprocessed is not a predetermined size and transform basis candidates(second transform basis candidates) in the case where the size of acurrent block to be processed is the predetermined size. It is to benoted that the first transform basis candidates and the second transformbasis candidates may include the same one or more transform bases.

Effects, Etc., of Variation 5 of Embodiment 1

As described above, with encoder 100 and decoder 200 according toVariation 5 of Embodiment 1, it is possible to remove the mutualdependency between the first transform basis candidates and the secondtransform basis candidates in Variation 4. For example, the secondtransform basis candidates can include one or more transform bases whichare not included in the first transform basis candidates. Accordingly,in this variation, it is possible to increase flexibility indetermination of transform basis candidates, thereby enabling use oftransform basis candidates more suitable for block sizes than those inVariation 4.

Other Variations of Embodiment 1

Although encoders and decoders according to one or more aspects of thepresent disclosure have been explained based on the above embodiment andthe variations thereof, the present disclosure is not limited to theembodiment and the variations thereof. The one or more aspects of thepresent disclosure may encompass embodiments obtainable by adding, toany of the embodiment and the variations thereof, various kinds ofmodifications that a person skilled in the art would arrive at withoutdeviating from the scope of the present disclosure and embodimentsconfigurable by arbitrarily combining constituent elements in differentembodiments.

For example, although switching between the fixed basis or bases and theselected basis is made according to the size of the current block ineach of Embodiment 1 and Variations 1 to 3 thereof, this is anon-limiting example. For example, switching between the fixed basis orbases and the selected basis may be made based also on the luminance andchrominance of luminance and chrominance blocks in addition to the sizesthereof. More specifically, for example as in the conventional art, abasis DST-VII may be fixedly used for a 4×4 luminance block in intraprediction. In other words, in the case of a luminance block or achrominance block in inter prediction, a basis selected from among aplurality of bases may be used irrespective of the size thereof.

Although Embodiment 1 and the variations thereof have been describedtaking orthogonal transform bases as examples, it is to be noted thatfrequency transforms are not limited to the orthogonal transforms.

It is to be noted that, as illustrated in FIG. 30, Variation 4 ofEmbodiment 1 indicates non-limiting examples of cases in which DCT2,DCT5, DCT8, DST1, and DST7 are used as the plurality of first transformbasis candidates, and either DCT2, DCT8, DST1, and DST7, or DCT2 andDCT5 are used as the one or more second transform basis candidates. Forexample, bases of types 1 to 8 defined based on boundary conditions andsymmetry may be used in each of DCTs and DSTs as the plurality of firsttransform basis candidates.

It is to be noted that the transform bases included in the firsttransform basis candidates and the second transform basis candidates donot always need to conform the DCT and DST shapes, and may be baseshaving any other shapes with properties similar to the properties of theDCT and DST shapes. In addition, the transform bases may be an eigenvector (Karhunen-Loeve transform (KLT)) which can be obtained by maincomponent analysis, or bases (for incomplete transform (IT)) which skipa transform process only in a particular direction in a two-dimensionaltransform.

It is to be noted that the transform bases may be separable ornon-separable. In addition, the transform process in which the transformbases are used may be one-dimensional transform or two-dimensionaltransform. In addition, when separable transform is applied, a basis maybe selected independently in each of the horizontal direction and thevertical direction. In this case, there may be some restrictions oncombinations of a basis in the horizontal direction and a basis in thevertical direction. For example, DCT2 may be used only when it is usedboth in the horizontal direction and in the vertical direction.

It is to be noted that degrees of reduction in processing amount varyeven when fast computation methods are present. For this reason, a basiswhich is determined to require a large processing amount may be excludedfrom transform basis candidates even when a fast computation method ispresent. For example, DST7 and DCT8 allow use of a fast computationmethod for a small block size, the processing amount is larger than theprocessing amount in the case where DCT2 is used. For this reason, thebases of DST7 and DCT8 are not always included in first transform basiscandidates.

Hereinafter, specific examples of combinations of a predeterminedcondition and transform basis candidates are listed.

(1)

Predetermined condition: the block size is larger than 8

First transform basis candidates: DCT2, DCT5, DCT8, DST1, and DST7

Second transform basis candidate: DCT2

In this combination, when the size of a current block to beencoded/decoded is a size of 8 or less (for example, 4 or 8), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT5,DCT8, DST1, and DST7. In addition, when the size of a current block tobe encoded/decoded is a size larger than 8 (for example, 16 or 32), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2.

(2)

Predetermined condition: the block size is larger than 8

First transform basis candidates: DCT2, DCT5, DCT4, DST1, and DST4

Second transform basis candidates: DCT2, DCT4, and DST4

In this combination, when the size of a current block to beencoded/decoded is a size of 8 or less (for example, 4 or 8), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT5,DCT4, DST1, and DST4. In addition, when the size of a current block tobe encoded/decoded is a size larger than 8 (for example, 16 or 32), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT3,and DST4.

(3)

Predetermined condition: the block size is larger than 16

Second transform basis candidates: DCT2, DCT4, DST4, and IT

Second transform basis candidates: DCT2 and IT

In this combination, when the size of a current block to beencoded/decoded is a size of 16 or less (for example, 4, 8, or 16), atransform process or inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT4,DST4, and IT. In addition, when the size of a current block to beencoded/decoded is a size larger than 16 (for example, 32), a transformprocess or an inverse transform process is performed on the currentblock using a transform basis or an inverse transform basis included intransform bases or inverse transform bases of DCT2 and IT.

(4)

Predetermined condition: the block size is larger than 16

First transform basis candidates: DCT2, DCT4, DST4, and IT

Second transform basis candidate: DCT2

In this combination, when the size of a current block to beencoded/decoded is a size of 16 or less (for example, 4, 8, or 16), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT4,DST4, and IT. In addition, when the size of a current block to beencoded/decoded is a size larger than 16 (for example, 32), a transformprocess or an inverse transform process is performed on the currentblock using a transform basis or an inverse transform basis of DCT2.

(5)

Predetermined condition: the block size is larger than 16

First transform basis candidates: DCT2, DCT4, and DST4

Second transform basis candidate: DCT2

In this combination, when the size of a current block to beencoded/decoded is a size of 16 or less (for example, 4, 8, or 16), atransform process or an inverse transform process is performed on thecurrent block using a transform basis or an inverse transform basisincluded in transform bases or inverse transform bases of DCT2, DCT4,and DST4. In addition, when the size of a current block to beencoded/decoded is a size larger than 16 (for example, 32), a transformprocess or an inverse transform process is performed on the currentblock using a transform basis or an inverse transform basis of DCT2.

It is to be noted that the plurality of combinations (1) to (5) areindicated as non-limiting examples. For example, in each of the abovecombinations, DCT8 and DST7 may be used instead of DCT4 and DST4. Inaddition, for example, 32 or 64 may be used instead of 8 and 16 as blocksizes in predetermined conditions.

Embodiment 2

As described in each of the above embodiments, each functional block cantypically be realized as an MPU and memory, for example. Moreover,processes performed by each of the functional blocks are typicallyrealized by a program execution unit, such as a processor, reading andexecuting software (a program) recorded on a recording medium such asROM. The software may be distributed via, for example, downloading, andmay be recorded on a recording medium such as semiconductor memory anddistributed. Note that each functional block can, of course, also berealized as hardware (dedicated circuit).

Moreover, the processing described in each of the embodiments may berealized via integrated processing using a single apparatus (system),and, alternatively, may be realized via decentralized processing using aplurality of apparatuses. Moreover, the processor that executes theabove-described program may be a single processor or a plurality ofprocessors. In other words, integrated processing may be performed, and,alternatively, decentralized processing may be performed.

Embodiments of the present disclosure are not limited to the aboveexemplary embodiments; various modifications may be made to theexemplary embodiments, the results of which are also included within thescope of the embodiments of the present disclosure.

Next, application examples of the moving picture encoding method (imageencoding method) and the moving picture decoding method (image decodingmethod) described in each of the above embodiments and a system thatemploys the same will be described. The system is characterized asincluding an image encoder that employs the image encoding method, animage decoder that employs the image decoding method, and an imageencoder/decoder that includes both the image encoder and the imagedecoder. Other configurations included in the system may be modified ona case-by-case basis.

[Usage Examples]

FIG. 34 illustrates an overall configuration of content providing systemex100 for implementing a content distribution service. The area in whichthe communication service is provided is divided into cells of desiredsizes, and base stations ex106, ex107, ex108, ex109, and ex110, whichare fixed wireless stations, are located in respective cells.

In content providing system ex100, devices including computer ex111,gaming device ex112, camera ex113, home appliance ex114, and smartphoneex115 are connected to internet ex101 via internet service providerex102 or communications network ex104 and base stations ex106 throughex110. Content providing system ex100 may combine and connect anycombination of the above elements. The devices may be directly orindirectly connected together via a telephone network or near fieldcommunication rather than via base stations ex106 through ex110, whichare fixed wireless stations. Moreover, streaming server ex103 isconnected to devices including computer ex111, gaming device ex112,camera ex113, home appliance ex114, and smartphone ex115 via, forexample, internet ex101. Streaming server ex103 is also connected to,for example, a terminal in a hotspot in airplane ex117 via satelliteex116.

Note that instead of base stations ex106 through ex110, wireless accesspoints or hotspots may be used. Streaming server ex103 may be connectedto communications network ex104 directly instead of via internet ex101or internet service provider ex102, and may be connected to airplaneex117 directly instead of via satellite ex116.

Camera ex113 is a device capable of capturing still images and video,such as a digital camera. Smartphone ex115 is a smartphone device,cellular phone, or personal handyphone system (PHS) phone that canoperate under the mobile communications system standards of the typical2G, 3G, 3.9G, and 4G systems, as well as the next-generation 5G system.

Home appliance ex118 is, for example, a refrigerator or a deviceincluded in a home fuel cell cogeneration system.

In content providing system ex100, a terminal including an image and/orvideo capturing function is capable of, for example, live streaming byconnecting to streaming server ex103 via, for example, base stationex106. When live streaming, a terminal (e.g., computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, orairplane ex117) performs the encoding processing described in the aboveembodiments on still-image or video content captured by a user via theterminal, multiplexes video data obtained via the encoding and audiodata obtained by encoding audio corresponding to the video, andtransmits the obtained data to streaming server ex103. In other words,the terminal functions as the image encoder according to one aspect ofthe present disclosure.

Streaming server ex103 streams transmitted content data to clients thatrequest the stream. Client examples include computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, andterminals inside airplane ex117, which are capable of decoding theabove-described encoded data. Devices that receive the streamed datadecode and reproduce the received data. In other words, the devices eachfunction as the image decoder according to one aspect of the presentdisclosure.

[Decentralized Processing]

Streaming server ex103 may be realized as a plurality of servers orcomputers between which tasks such as the processing, recording, andstreaming of data are divided. For example, streaming server ex103 maybe realized as a content delivery network (CDN) that streams content viaa network connecting multiple edge servers located throughout the world.In a CDN, an edge server physically near the client is dynamicallyassigned to the client. Content is cached and streamed to the edgeserver to reduce load times. In the event of, for example, some kind ofan error or a change in connectivity due to, for example, a spike intraffic, it is possible to stream data stably at high speeds since it ispossible to avoid affected parts of the network by, for example,dividing the processing between a plurality of edge servers or switchingthe streaming duties to a different edge server, and continuingstreaming.

Decentralization is not limited to just the division of processing forstreaming; the encoding of the captured data may be divided between andperformed by the terminals, on the server side, or both. In one example,in typical encoding, the processing is performed in two loops. The firstloop is for detecting how complicated the image is on a frame-by-frameor scene-by-scene basis, or detecting the encoding load. The second loopis for processing that maintains image quality and improves encodingefficiency. For example, it is possible to reduce the processing load ofthe terminals and improve the quality and encoding efficiency of thecontent by having the terminals perform the first loop of the encodingand having the server side that received the content perform the secondloop of the encoding. In such a case, upon receipt of a decodingrequest, it is possible for the encoded data resulting from the firstloop performed by one terminal to be received and reproduced on anotherterminal in approximately real time. This makes it possible to realizesmooth, real-time streaming.

In another example, camera ex113 or the like extracts a feature amountfrom an image, compresses data related to the feature amount asmetadata, and transmits the compressed metadata to a server. Forexample, the server determines the significance of an object based onthe feature amount and changes the quantization accuracy accordingly toperform compression suitable for the meaning of the image. Featureamount data is particularly effective in improving the precision andefficiency of motion vector prediction during the second compressionpass performed by the server. Moreover, encoding that has a relativelylow processing load, such as variable length coding (VLC), may behandled by the terminal, and encoding that has a relatively highprocessing load, such as context-adaptive binary arithmetic coding(CABAC), may be handled by the server.

In yet another example, there are instances in which a plurality ofvideos of approximately the same scene are captured by a plurality ofterminals in, for example, a stadium, shopping mall, or factory. In sucha case, for example, the encoding may be decentralized by dividingprocessing tasks between the plurality of terminals that captured thevideos and, if necessary, other terminals that did not capture thevideos and the server, on a per-unit basis. The units may be, forexample, groups of pictures (GOP), pictures, or tiles resulting fromdividing a picture. This makes it possible to reduce load times andachieve streaming that is closer to real-time.

Moreover, since the videos are of approximately the same scene,management and/or instruction may be carried out by the server so thatthe videos captured by the terminals can be cross-referenced. Moreover,the server may receive encoded data from the terminals, change referencerelationship between items of data or correct or replace picturesthemselves, and then perform the encoding. This makes it possible togenerate a stream with increased quality and efficiency for theindividual items of data.

Moreover, the server may stream video data after performing transcodingto convert the encoding format of the video data. For example, theserver may convert the encoding format from MPEG to VP, and may convertH.264 to H.265.

In this way, encoding can be performed by a terminal or one or moreservers. Accordingly, although the device that performs the encoding isreferred to as a “server” or “terminal” in the following description,some or all of the processes performed by the server may be performed bythe terminal, and likewise some or all of the processes performed by theterminal may be performed by the server. This also applies to decodingprocesses.

[3D, Multi-Angle]

In recent years, usage of images or videos combined from images orvideos of different scenes concurrently captured or the same scenecaptured from different angles by a plurality of terminals such ascamera ex113 and/or smartphone ex115 has increased. Videos captured bythe terminals are combined based on, for example, theseparately-obtained relative positional relationship between theterminals, or regions in a video having matching feature points.

In addition to the encoding of two-dimensional moving pictures, theserver may encode a still image based on scene analysis of a movingpicture either automatically or at a point in time specified by theuser, and transmit the encoded still image to a reception terminal.Furthermore, when the server can obtain the relative positionalrelationship between the video capturing terminals, in addition totwo-dimensional moving pictures, the server can generatethree-dimensional geometry of a scene based on video of the same scenecaptured from different angles. Note that the server may separatelyencode three-dimensional data generated from, for example, a pointcloud, and may, based on a result of recognizing or tracking a person orobject using three-dimensional data, select or reconstruct and generatea video to be transmitted to a reception terminal from videos capturedby a plurality of terminals.

This allows the user to enjoy a scene by freely selecting videoscorresponding to the video capturing terminals, and allows the user toenjoy the content obtained by extracting, from three-dimensional datareconstructed from a plurality of images or videos, a video from aselected viewpoint. Furthermore, similar to with video, sound may berecorded from relatively different angles, and the server may multiplex,with the video, audio from a specific angle or space in accordance withthe video, and transmit the result.

In recent years, content that is a composite of the real world and avirtual world, such as virtual reality (VR) and augmented reality (AR)content, has also become popular. In the case of VR images, the servermay create images from the viewpoints of both the left and right eyesand perform encoding that tolerates reference between the two viewpointimages, such as multi-view coding (MVC), and, alternatively, may encodethe images as separate streams without referencing. When the images aredecoded as separate streams, the streams may be synchronized whenreproduced so as to recreate a virtual three-dimensional space inaccordance with the viewpoint of the user.

In the case of AR images, the server superimposes virtual objectinformation existing in a virtual space onto camera informationrepresenting a real-world space, based on a three-dimensional positionor movement from the perspective of the user. The decoder may obtain orstore virtual object information and three-dimensional data, generatetwo-dimensional images based on movement from the perspective of theuser, and then generate superimposed data by seamlessly connecting theimages. Alternatively, the decoder may transmit, to the server, motionfrom the perspective of the user in addition to a request for virtualobject information, and the server may generate superimposed data basedon three-dimensional data stored in the server in accordance with thereceived motion, and encode and stream the generated superimposed datato the decoder. Note that superimposed data includes, in addition to RGBvalues, an α value indicating transparency, and the server sets the αvalue for sections other than the object generated fromthree-dimensional data to, for example, 0, and may perform the encodingwhile those sections are transparent. Alternatively, the server may setthe background to a predetermined RGB value, such as a chroma key, andgenerate data in which areas other than the object are set as thebackground.

Decoding of similarly streamed data may be performed by the client(i.e., the terminals), on the server side, or divided therebetween. Inone example, one terminal may transmit a reception request to a server,the requested content may be received and decoded by another terminal,and a decoded signal may be transmitted to a device having a display. Itis possible to reproduce high image quality data by decentralizingprocessing and appropriately selecting content regardless of theprocessing ability of the communications terminal itself. In yet anotherexample, while a TV, for example, is receiving image data that is largein size, a region of a picture, such as a tile obtained by dividing thepicture, may be decoded and displayed on a personal terminal orterminals of a viewer or viewers of the TV. This makes it possible forthe viewers to share a big-picture view as well as for each viewer tocheck his or her assigned area or inspect a region in further detail upclose.

In the future, both indoors and outdoors, in situations in which aplurality of wireless connections are possible over near, mid, and fardistances, it is expected to be able to seamlessly receive content evenwhen switching to data appropriate for the current connection, using astreaming system standard such as MPEG-DASH. With this, the user canswitch between data in real time while freely selecting a decoder ordisplay apparatus including not only his or her own terminal, but also,for example, displays disposed indoors or outdoors. Moreover, based on,for example, information on the position of the user, decoding can beperformed while switching which terminal handles decoding and whichterminal handles the displaying of content. This makes it possible to,while in route to a destination, display, on the wall of a nearbybuilding in which a device capable of displaying content is embedded oron part of the ground, map information while on the move. Moreover, itis also possible to switch the bit rate of the received data based onthe accessibility to the encoded data on a network, such as when encodeddata is cached on a server quickly accessible from the receptionterminal or when encoded data is copied to an edge server in a contentdelivery service.

[Scalable Encoding]

The switching of content will be described with reference to a scalablestream, illustrated in FIG. 35, that is compression coded viaimplementation of the moving picture encoding method described in theabove embodiments. The server may have a configuration in which contentis switched while making use of the temporal and/or spatial scalabilityof a stream, which is achieved by division into and encoding of layers,as illustrated in FIG. 35. Note that there may be a plurality ofindividual streams that are of the same content but different quality.In other words, by determining which layer to decode up to based oninternal factors, such as the processing ability on the decoder side,and external factors, such as communication bandwidth, the decoder sidecan freely switch between low resolution content and high resolutioncontent while decoding. For example, in a case in which the user wantsto continue watching, at home on a device such as a TV connected to theinternet, a video that he or she had been previously watching onsmartphone ex115 while on the move, the device can simply decode thesame stream up to a different layer, which reduces server side load.

Furthermore, in addition to the configuration described above in whichscalability is achieved as a result of the pictures being encoded perlayer and the enhancement layer is above the base layer, the enhancementlayer may include metadata based on, for example, statisticalinformation on the image, and the decoder side may generate high imagequality content by performing super-resolution imaging on a picture inthe base layer based on the metadata. Super-resolution imaging may beimproving the SN ratio while maintaining resolution and/or increasingresolution. Metadata includes information for identifying a linear or anon-linear filter coefficient used in super-resolution processing, orinformation identifying a parameter value in filter processing, machinelearning, or least squares method used in super-resolution processing.

Alternatively, a configuration in which a picture is divided into, forexample, tiles in accordance with the meaning of, for example, an objectin the image, and on the decoder side, only a partial region is decodedby selecting a tile to decode, is also acceptable. Moreover, by storingan attribute about the object (person, car, ball, etc.) and a positionof the object in the video (coordinates in identical images) asmetadata, the decoder side can identify the position of a desired objectbased on the metadata and determine which tile or tiles include thatobject. For example, as illustrated in FIG. 36, metadata is stored usinga data storage structure different from pixel data such as an SEImessage in HEVC. This metadata indicates, for example, the position,size, or color of the main object.

Moreover, metadata may be stored in units of a plurality of pictures,such as stream, sequence, or random access units. With this, the decoderside can obtain, for example, the time at which a specific personappears in the video, and by fitting that with picture unit information,can identify a picture in which the object is present and the positionof the object in the picture.

[Web Page Optimization]

FIG. 37 illustrates an example of a display screen of a web page on, forexample, computer ex111. FIG. 38 illustrates an example of a displayscreen of a web page on, for example, smartphone ex115. As illustratedin FIG. 37 and FIG. 38, a web page may include a plurality of imagelinks which are links to image content, and the appearance of the webpage differs depending on the device used to view the web page. When aplurality of image links are viewable on the screen, until the userexplicitly selects an image link, or until the image link is in theapproximate center of the screen or the entire image link fits in thescreen, the display apparatus (decoder) displays, as the image links,still images included in the content or I pictures, displays video suchas an animated gif using a plurality of still images or I pictures, forexample, or receives only the base layer and decodes and displays thevideo.

When an image link is selected by the user, the display apparatusdecodes giving the highest priority to the base layer. Note that ifthere is information in the HTML code of the web page indicating thatthe content is scalable, the display apparatus may decode up to theenhancement layer. Moreover, in order to guarantee real timereproduction, before a selection is made or when the bandwidth isseverely limited, the display apparatus can reduce delay between thepoint in time at which the leading picture is decoded and the point intime at which the decoded picture is displayed (that is, the delaybetween the start of the decoding of the content to the displaying ofthe content) by decoding and displaying only forward reference pictures(I picture, P picture, forward reference B picture). Moreover, thedisplay apparatus may purposely ignore the reference relationshipbetween pictures and coarsely decode all B and P pictures as forwardreference pictures, and then perform normal decoding as the number ofpictures received over time increases.

[Autonomous Driving]

When transmitting and receiving still image or video data such two- orthree-dimensional map information for autonomous driving or assisteddriving of an automobile, the reception terminal may receive, inaddition to image data belonging to one or more layers, information on,for example, the weather or road construction as metadata, and associatethe metadata with the image data upon decoding. Note that metadata maybe assigned per layer and, alternatively, may simply be multiplexed withthe image data.

In such a case, since the automobile, drone, airplane, etc., includingthe reception terminal is mobile, the reception terminal can seamlesslyreceive and decode while switching between base stations among basestations ex106 through ex110 by transmitting information indicating theposition of the reception terminal upon reception request. Moreover, inaccordance with the selection made by the user, the situation of theuser, or the bandwidth of the connection, the reception terminal candynamically select to what extent the metadata is received or to whatextent the map information, for example, is updated.

With this, in content providing system ex100, the client can receive,decode, and reproduce, in real time, encoded information transmitted bythe user.

[Streaming of Individual Content]

In content providing system ex100, in addition to high image quality,long content distributed by a video distribution entity, unicast ormulticast streaming of low image quality, short content from anindividual is also possible. Moreover, such content from individuals islikely to further increase in popularity. The server may first performediting processing on the content before the encoding processing inorder to refine the individual content. This may be achieved with, forexample, the following configuration.

In real-time while capturing video or image content or after the contenthas been captured and accumulated, the server performs recognitionprocessing based on the raw or encoded data, such as capture errorprocessing, scene search processing, meaning analysis, and/or objectdetection processing. Then, based on the result of the recognitionprocessing, the server—either when prompted or automatically—edits thecontent, examples of which include: correction such as focus and/ormotion blur correction; removing low-priority scenes such as scenes thatare low in brightness compared to other pictures or out of focus; objectedge adjustment; and color tone adjustment. The server encodes theedited data based on the result of the editing. It is known thatexcessively long videos tend to receive fewer views. Accordingly, inorder to keep the content within a specific length that scales with thelength of the original video, the server may, in addition to thelow-priority scenes described above, automatically clip out scenes withlow movement based on an image processing result. Alternatively, theserver may generate and encode a video digest based on a result of ananalysis of the meaning of a scene.

Note that there are instances in which individual content may includecontent that infringes a copyright, moral right, portrait rights, etc.Such an instance may lead to an unfavorable situation for the creator,such as when content is shared beyond the scope intended by the creator.Accordingly, before encoding, the server may, for example, edit imagesso as to blur faces of people in the periphery of the screen or blur theinside of a house, for example. Moreover, the server may be configuredto recognize the faces of people other than a registered person inimages to be encoded, and when such faces appear in an image, forexample, apply a mosaic filter to the face of the person. Alternatively,as pre- or post-processing for encoding, the user may specify, forcopyright reasons, a region of an image including a person or a regionof the background be processed, and the server may process the specifiedregion by, for example, replacing the region with a different image orblurring the region. If the region includes a person, the person may betracked in the moving picture, and the head region may be replaced withanother image as the person moves.

Moreover, since there is a demand for real-time viewing of contentproduced by individuals, which tends to be small in data size, thedecoder first receives the base layer as the highest priority andperforms decoding and reproduction, although this may differ dependingon bandwidth. When the content is reproduced two or more times, such aswhen the decoder receives the enhancement layer during decoding andreproduction of the base layer and loops the reproduction, the decodermay reproduce a high image quality video including the enhancementlayer. If the stream is encoded using such scalable encoding, the videomay be low quality when in an unselected state or at the start of thevideo, but it can offer an experience in which the image quality of thestream progressively increases in an intelligent manner. This is notlimited to just scalable encoding; the same experience can be offered byconfiguring a single stream from a low quality stream reproduced for thefirst time and a second stream encoded using the first stream as areference.

[Other Usage Examples]

The encoding and decoding may be performed by LSI ex500, which istypically included in each terminal. LSI ex500 may be configured of asingle chip or a plurality of chips. Software for encoding and decodingmoving pictures may be integrated into some type of a recording medium(such as a CD-ROM, a flexible disk, or a hard disk) that is readable by,for example, computer ex111, and the encoding and decoding may beperformed using the software. Furthermore, when smartphone ex115 isequipped with a camera, the video data obtained by the camera may betransmitted. In this case, the video data is coded by LSI ex500 includedin smartphone ex115.

Note that LSI ex500 may be configured to download and activate anapplication. In such a case, the terminal first determines whether it iscompatible with the scheme used to encode the content or whether it iscapable of executing a specific service. When the terminal is notcompatible with the encoding scheme of the content or when the terminalis not capable of executing a specific service, the terminal firstdownloads a codec or application software then obtains and reproducesthe content.

Aside from the example of content providing system ex100 that usesinternet ex101, at least the moving picture encoder (image encoder) orthe moving picture decoder (image decoder) described in the aboveembodiments may be implemented in a digital broadcasting system. Thesame encoding processing and decoding processing may be applied totransmit and receive broadcast radio waves superimposed with multiplexedaudio and video data using, for example, a satellite, even though thisis geared toward multicast whereas unicast is easier with contentproviding system ex100.

[Hardware Configuration]

FIG. 39 illustrates smartphone ex115. FIG. 40 illustrates aconfiguration example of smartphone ex115. Smartphone ex115 includesantenna ex450 for transmitting and receiving radio waves to and frombase station ex110, camera ex465 capable of capturing video and stillimages, and display ex458 that displays decoded data, such as videocaptured by camera ex465 and video received by antenna ex450. Smartphoneex115 further includes user interface ex466 such as a touch panel, audiooutput unit ex457 such as a speaker for outputting speech or otheraudio, audio input unit ex456 such as a microphone for audio input,memory ex467 capable of storing decoded data such as captured video orstill images, recorded audio, received video or still images, and mail,as well as decoded data, and slot ex464 which is an interface for SIMex468 for authorizing access to a network and various data. Note thatexternal memory may be used instead of memory ex467.

Moreover, main controller ex460 which comprehensively controls displayex458 and user interface ex466, power supply circuit ex461, userinterface input controller ex462, video signal processor ex455, camerainterface ex463, display controller ex459, modulator/demodulator ex452,multiplexer/demultiplexer ex453, audio signal processor ex454, slotex464, and memory ex467 are connected via bus ex470.

When the user turns the power button of power supply circuit ex461 on,smartphone ex115 is powered on into an operable state by each componentbeing supplied with power from a battery pack.

Smartphone ex115 performs processing for, for example, calling and datatransmission, based on control performed by main controller ex460, whichincludes a CPU, ROM, and RAM. When making calls, an audio signalrecorded by audio input unit ex456 is converted into a digital audiosignal by audio signal processor ex454, and this is applied with spreadspectrum processing by modulator/demodulator ex452 and digital-analogconversion and frequency conversion processing by transmitter/receiverex451, and then transmitted via antenna ex450. The received data isamplified, frequency converted, and analog-digital converted, inversespread spectrum processed by modulator/demodulator ex452, converted intoan analog audio signal by audio signal processor ex454, and then outputfrom audio output unit ex457. In data transmission mode, text,still-image, or video data is transmitted by main controller ex460 viauser interface input controller ex462 as a result of operation of, forexample, user interface ex466 of the main body, and similar transmissionand reception processing is performed. In data transmission mode, whensending a video, still image, or video and audio, video signal processorex455 compression encodes, via the moving picture encoding methoddescribed in the above embodiments, a video signal stored in memoryex467 or a video signal input from camera ex465, and transmits theencoded video data to multiplexer/demultiplexer ex453. Moreover, audiosignal processor ex454 encodes an audio signal recorded by audio inputunit ex456 while camera ex465 is capturing, for example, a video orstill image, and transmits the encoded audio data tomultiplexer/demultiplexer ex453. Multiplexer/demultiplexer ex453multiplexes the encoded video data and encoded audio data using apredetermined scheme, modulates and converts the data usingmodulator/demodulator (modulator/demodulator circuit) ex452 andtransmitter/receiver ex451, and transmits the result via antenna ex450.

When video appended in an email or a chat, or a video linked from a webpage, for example, is received, in order to decode the multiplexed datareceived via antenna ex450, multiplexer/demultiplexer ex453demultiplexes the multiplexed data to divide the multiplexed data into abitstream of video data and a bitstream of audio data, supplies theencoded video data to video signal processor ex455 via synchronous busex470, and supplies the encoded audio data to audio signal processorex454 via synchronous bus ex470. Video signal processor ex455 decodesthe video signal using a moving picture decoding method corresponding tothe moving picture encoding method described in the above embodiments,and video or a still image included in the linked moving picture file isdisplayed on display ex458 via display controller ex459. Moreover, audiosignal processor ex454 decodes the audio signal and outputs audio fromaudio output unit ex457. Note that since real-time streaming is becomingmore and more popular, there are instances in which reproduction of theaudio may be socially inappropriate depending on the user's environment.Accordingly, as an initial value, a configuration in which only videodata is reproduced, i.e., the audio signal is not reproduced, ispreferable. Audio may be synchronized and reproduced only when an input,such as when the user clicks video data, is received.

Although smartphone ex115 was used in the above example, threeimplementations are conceivable: a transceiver terminal including bothan encoder and a decoder; a transmitter terminal including only anencoder; and a receiver terminal including only a decoder. Further, inthe description of the digital broadcasting system, an example is givenin which multiplexed data obtained as a result of video data beingmultiplexed with, for example, audio data, is received or transmitted,but the multiplexed data may be video data multiplexed with data otherthan audio data, such as text data related to the video. Moreover, thevideo data itself rather than multiplexed data maybe received ortransmitted.

Although main controller ex460 including a CPU is described ascontrolling the encoding or decoding processes, terminals often includeGPUs. Accordingly, a configuration is acceptable in which a large areais processed at once by making use of the performance ability of the GPUvia memory shared by the CPU and GPU or memory including an address thatis managed so as to allow common usage by the CPU and GPU. This makes itpossible to shorten encoding time, maintain the real-time nature of thestream, and reduce delay. In particular, processing relating to motionestimation, deblocking filtering, sample adaptive offset (SAO), andtransformation/quantization can be effectively carried out by the GPUinstead of the CPU in units of, for example pictures, all at once.

Although only some exemplary embodiments of the present disclosure havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present disclosure. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to, for example, televisionreceivers, digital video recorders, car navigation systems, mobilephones, digital cameras, digital video cameras, etc.

What is claimed is:
 1. A decoder which inverse-transforms a currentblock to be decoded in an encoded image to decode the current block, thedecoder comprising: circuitry; and memory, wherein the circuitry, usingthe memory: determines a first group of candidates andinverse-transforms the current block using an inverse transform basiswhich is one of the candidates included in the first group of candidatesdetermined, when the current block has a size of 16 or less; anddetermines one second candidate outside of the first group of candidatesand inverse-transforms the current block using an inverse transformbasis which is the second candidate determined, when the current blockhas a size larger than
 16. 2. A decoding method for inverse-transforminga current block to be decoded in an encoded image to decode the currentblock, the decoding method comprising: determining a first group ofcandidates and inverse-transforming the current block using an inversetransform basis which is one of the candidates included in the firstgroup of candidates determined, when the current block has a size of 16or less; and determining one second candidate outside of the first groupof candidates and inverse-transforming the current block using aninverse transform basis which is the second candidate determined, whenthe current block has a size larger than 16.